I don’t think it’s inappropriate for him to say “something.” I think it’s inappropriate for him to address this 20 year old as if he, the boyfriend, is the Head of the Household issuing a direct order that will be obeyed. The OP has already stated that: a. the SO addressed the son in a parent/child manner, and b. the question she is most concerned about is whether this same situation may arise in her parenting of her younger child, which seems like a perfectly legitimate concern, and one you are ignoring in favor of trying to make the SO sound like he was having a manly heart-to-heart on a grownup level.
So, since the OP is concerned about the possiblity of the boyfriend intervening and/or interfering in her parenting her younger child, that seems like a logical place to focus our attention.
My feeling is that SO has the impression that Mom just isn’t tough enough and needs someone to step up. Or “Man Up”. Or surrogate parent. Or teach her by example How to Handle These Kids. And as several people have pointed out, it’s not his place to parent either of her kids, and his attempt at laying down the law with a 20 year old is disrespectful and unacceptable.
We who forget the past actions of others are doomed to allow others to repeat them. I pointed out a nascent derail in the hopes of heading it off before yet another thread was ruined.
As opposed to someone who has no children whatsoever?
Papergirl sounds exactly like my exwife and she’s also correct, the SO was out of bounds and should probably be shown the door and invited to never return.
I will admit to some bias though, having kids and having to have dealt with this issue with a past girlfriend
Concur. Then again, she could also gently tell him why this bothered her, express appreciation for his good intentions, and then consider the subject closed. Not quite as dramatic, though. 