Probably, but is there any other way to answer the OP?
Some could try not assuming the OP sees something weird that needs to be fixed. 
Sure, and some people here have tried just that. But that view, that the OP’s relative is perfectly content and has everything in his life exactly the way he wants it, is just as much of a projection and assumption.
I was going to write a long message detailing my own experience being not unlike the “adult child” mentioned in the OP. But really it can all be summed up thusly:
Shyness is a bitch***
Shyness alone could explain his apparent immaturity. Regular social activity keeps people sharp and relatively sophisticated. When an individual is socially isolated, they don’t learn what’s socially “cool” and “uncool”, so they can come across as awkward and not really “with” it.
So perhaps there’s nothing more going on besides the obvious, Ken001. And I don’t really get why you have to do more than simply be his friend to “fix” him. Like, if you think he needs stimulation, expose him to new things. If you think he’s lonely, spend more time with him. Coming up with a label for him isn’t going to make him a happier person.
He doesn’t need a diagnosis to be helped by a professional. But he does need to want that kind of help. Unless he brings up the idea, I wouldn’t mention it.
Just a quick note to acknowledge that my relative doesn’t have a critical condition. He isn’t facing a crisis. The very act of writing my thoughts here magnifies the reality - its actually a subtle personality issue.
He isn’t stupid, just sorta redneck narrow. I care about the man and hope to encourage him to find the spark in life.
Thank you everyone for your thoughts including those who think I should not interfere. Its a good point. This chaps innocence/naivety has worried me for 20 years and the best I’ve done is to treat him with dignity and kindness. And maybe that is enough.
Monstro and Tripolar have suggested he should be introduced to new people and situations. I totally agree.
He has a touch of the fundamentalist Christianity and was going to church for a while. However he only made one or two friends, which in retrospect is surprising. Probably lack of confidence and shyness.
My big idea was for him to work as a volunteer at the Habitat for Humanity charity shop. It is a busy successful place and some of the staff/volunteers are people from the edges of society. People who wouldn’t get a job elsewhere. I’ve helped there a bit myself and I know he would enjoy it.
However when I suggested the idea his instant response was No. I attempted to discuss it and he just wandered away.
Yes, what Nava said. And I never said he had “normal social skills” – in fact, he had almost none. He was very awkward. I said he could function normally, meaning take care of himself, hold a job etc. A nice guy, but believe me, everyone who knew him saw an innocent child rather than an adult.
Or maybe he didn’t mesh with most of the people there, which is why he stopped going. Friendship is a two-way street, and most people don’t really get along well with 50-year-old eccentrics. What does he say about the people there? Did he like them or he is just “meh” about them?
Maybe come up with a smaller idea–something that doesn’t involve any kind of commitment or “cold” interactions with strangers. When I picture myself working at a charity shop, I see me stressing over a cash register or having to explain stuff to customers. He could be having the same image in his head. He might prefer working with animals to people or helping individuals on his own terms rather than an organization’s. Like, does he have a truck? Maybe he could volunteer to help people move.
it’s really looking like you’re the one with the problem here. You can’t take a hint. He doesn’t need or want your “help”, so stop trying to stick your nose into his life.
I’m serious. Knock it off.
Yes, we know you’re serious. You’re also projecting heavily, without any actual knowledge of the situation. You came in to vilify the OP, and thus the OP has no reason to actually listen to you. When you treat people like crap, they have every reason not to do what you want.
If the guy doesn’t like it, he’s an adult and can tell him that he doesn’t like it–unless you really do think there’s something wrong with him that would prevent that.
The OP came in here for advice. You tried to turn it into a fight. That’s enough in my book to ignore what you have to say. Hell, it’s why I don’t ask for advice here, as there’s always someone.