Yes, you’d need to tell. Just take a page from the people who explain adoption to their children to get an idea of how to share the information, in an age appropriate way through the years.
In these days of open adoptions (and that’s basically what would be happening here, in the case of the boyfriend, just without the paperwork), step parents, etc. your kid wouldn’t be the only one of his peers who has more than two parents, and knows about it.
Hmmm
I am adopted and grew up always knowing. I don’t think medical history it thaaat important - the truely life altering diseases can be tested for early, the rest, meh, what will be will be.
Having said that, I think I would fall on the side of the kid should know. But I wouldn’t portray it as cheated - I would portray it more as a case of “taking a break”.
Realistically, I think a lot of people decide that the boyfriend will ‘raise the child as his own’ and it becomes a family secret that does or doesn’t come out into the open during the child’s lifetime.
I think the ideal thing to do is similar to what my parents did after adopting me, which is that they told me from such a young age that I was adopted, that it was never that big a deal to me. So right from being a toddler, they’d talk about how mum was with another man before the child was born, but she’s not with him anymore, and now the boyfriend is going to be his dad for the rest of his life.
What are you planning on doing for the other dynamics of the family? Will the mother’s parents know that the boyfriend isn’t the child’s natural father? And if so, what do they think/feel about that? What about the boyfriend’s parents? Will the rocker ever get to know that he has fathered a child? If so, how will he feel about that?
Opportunities present themselves. Kids ask all kinds of questions, all the time. Personally, I would just take the “never a secret” approach. My kid would always know that he had two “fathers”, a guy who helped Mommy make him, and Daddy, who loves him and takes care of him. Yeah, that will lead to more complicated questions later. Just answer them. Part of parenting is doing what’s best for your kid, even when it’s uncomfortable for you.
If anyone really thinks that parentage is a “useless factoid”, then why NOT share? The way I see it, it really is a “useless factoid” when it’s something you’ve always known. Much less so when it’s something you find out when you’re old enough to appreciate that you’ve been lied to all your life.
Yeah, personally, I admit that I’d be inclined not to tell the kid, unless there’s no way of hiding it—like if their biological father was a different race, or they had some medical issues that were going to make it impossible to ignore. Like they need to start screening relatives for a kidney transplant, or even something as mundane as the kid finding out their blood type, which would be impossible for the husband to have contributed to.
In any case, if I did want to tell the kid, I’d either have it be one of those “never a secret” things, or tell them when they were about college age. Call me a scheming cynic, but I don’t think “I’m not your ‘real’ father” is the kind of thing you’d want to tell an early teenager. A little too much potential drama fuel. 
Secrets are bad. Kids ave enough trouble rowing up sane in this world.
You need to let the pop star know - his name needs to go on the birth certificate.
You need to have any sort of contractual agreement paperwork drawn up [is he going to be paying any sort of child support or have him sign the authorization for someone else to adopt]
You fiancee needs to actually marry the mom adopt the kid. It is possible at that time to tell the kid as he is growing up that mommy and daddy are married, but his mother left his father and adoptive father wanted to be his father so much that he went to a judge and adopted him. Once he hits teendom, he will see the birth certificate as he is getting his drivers permit, so you can sit down and tell him as much of the story as he is ready for.
Wouldn’t the economic aspects also be relevant?
The child has a right to child support from the bio-father and (I assume this is nowadays the case in most jurisdictions) will be one of his heirs together with any other children. To ensure that the mother would need to get the paternity acknowledged.
Alternatively, if the boyfriend acknowledges the child as his, the child will have a claim to support from him and will inherit from him.
So the mother would also have a duty to the child to consider the respective economic prospects of the bio-father and boyfriend.
Wow, I’m so glad this thread has gotten this many responses so far! Brainstorming is fun.
Hmm, some interesting questions. I planned for the boyfriend’s name to go on the birth certificate, because the mother doesn’t intend to get the rocker entangled in child support (he couldn’t afford it and on top of that they live in different countries). I did plan for her to tell the rocker that he has a kid, because she is going to make him get medical records from his family - but now it’s just occurred to me that perhaps he wouldn’t be legally allowed to give her his medical records unless his name is on the birth certificate. Can someone tell me whether this would be legal? (He lives in Oakland, CA and she lives in London, for reference.)
Tell the kid! They’ll find out some way or another. And they’ll find out at the worst possible time. If you tell them, you can tell him at the time you think is best, and deal with how he feels.
It’s his father, how does he not have a right to know?
Of course since you are writing a story you can make some major drama by not telling him and then having him find out. 
If the biological father is of a different race, waiting till 18 may not make sense. There’s only so far you can go with “must be a recessive gene” in explaining away some things.
My mom told me absolutely nothing. She insisted my dad was my biological dad until the day she died, even though I had proof that there was no way he could be. I love my dad, who was honest with me: but I resented my mother for a long time because she couldn’t tell me the truth.
That’s from a real-life standpoint. Hope it helps.
I work in the medical field - my best understanding of American law is that if you request copies for yourself of your own medical records, you can do whatever you want with them. Post them on the Internet, make paper airplanes, wrap Christmas gifts with them. Dunno about passing on actual medical records of his family (UK law may vary, it wouldn’t be OK in the US I think?) but he could write up a history of his family’s illnesses I suspect.
How, exactly is she going to ‘make’ him do this medical record thing? Aren’t rock stars notoriously unwilling to acknowledge paternity?
If he’s going to be forced to acknowledge paternity then there is no way they can not tell the kid.
They are going to have to bite the bullet and confess that Mommy was a groupie, the sooner the better for the child. Start when he’s young, fill out the details as he gets older.
As a kid who’s birth certificate has always been a lie (correct bio dad never listed and then adopted dad added) the idea of lying to your kid and giving false info on a birth certificate is repulsive to me.
I say tel the kid the truth and give correct info on the birth certificate.
At some point, for medical reasons, the kid is going to need to know that his father is not his biological father. I think the most compassionate thing to do at that time is tell the child (when he/she is old enough) that the sperm was donated by another man, but the man who has raised the child is very much the child’s “real” father. Just let the sperm donor be anonymous.
The secret would definitely come out if rock dad had an eventful medical history, or if he had a parent or another child who had some sort of genetic disease. Unless adoptive dad takes that medical history and pretends its his own - when he can. It obviously wouldn’t work when the kid wanted to know how her grandma died young from cancer, when both her grandmothers are still alive.
Thank God I haven’t needed that kind of information, because it would be impossible to get. My dad said, “You know, it was the 60’s, it was a time of free love, etc…” In other words, he had no idea who it could be, but he didn’t care: bio or not, I was his daughter.
But knowing would have saved me a heck of a lot of embarrassment in high school biology.
Having had it happen, tell the child before some well meaning (or more likely, drunken score-settling) relative blurts it out.
There are people out there that will tell everyone the details of this stuff to anyone who will listen.
I have a friend who will tell anyone within ear shot that the decision to have a 4th child was based on the flip of a coin with her husband. One day I just may punch her when she is telling someone new about their 4th.
Another person that I had only just met told me her son was conceived in a hot tub and she and her husband were wasted and she didn’t seem to think either were a factor in the mental issues their son now had. ( It was a very uncomfortable moment.) I’m thinking that if you tell complete strangers the situation that you child(ren) was conceived in AND how fucked up your child THEN WHAT IN THE HELL ARE YOU SAYING TO CLOSE FRIENDS AND FAMILY? The kid will eventually beleive it all and if you keep on saying that the child is a pain in the ass or a flighty little thing, GUESS FUCKING WHAT…they become a PITA or Flighty LITTLE THING. sorry, sorry…I’ll step down from the clock tower.
Another person, whom I hadn’t realized use to be in my Girl Scout troop X years ago, told me her son was conceived while smoking weed AND that he had Turret’s. Therefore, turret’s was caused by dope. AFAIK, it is all genetics. When I made the connection she was in my former scout troup, I said something like, " Ummm You use to be smart. What the hell happened?"
With the abortion that is Reality TV, it reinforces this weird attention whore behavior.
SOOOOOO back on topic, unless the sperm donor dad looks alot like the adoptive dad, I am thinking a frank discussion or *You were Adopted *speech which would be lying and that would be paramount to the story line, because a Frank Discussion about Real Life Issues IS BORING AS ALL HELL and no one wants THE TRUTH because YOU CAN’T HANDLE THE TRUTH needs to be done. Lies and SEX sell.
There are genetic tics that kids who are adopted ( like me) realize They Just Don’t Farking Fit In With The Big Picture. And it isn’t BIG ASS things ok, it is Big Ass things, it is the daily things like, oooh, work ethics, mental attitude, how they OCD* their stuff, laughter, humor, food likes, hobbies and interests. If it is a Rocker, there is some music stuff floating in the DNA that will come out.
I would think a story arc could be the rocker’s point of view or advising him of the one night stand and his reaction to it.
*Everyone is OCD about their own stuff, man.
Interesting thing - if the child has some unexplainable trait - like freckles or red hair or brown eyes . . .
I think it would be cool for the boyfriend-dad to tell the kid. Talking about ‘girls’ and taking care of them. And how maybe he’s not the kids biological father - but he’ll always be his dad.
Not telling lends drama - but telling lends credibility.