What do you mean by seriously autistic? You said in another post he does tasks around the house and took care of your wife when she fell ill. A seriously autistic person would not be able to do that. A seriously autistic person would require constant adult supervision.
Are you sure he's not just under-socialized and addicted to porn and video games? How does he spend his days? He might simply be a western hikikomori.
I don't mean to pry, but I'm kinda curious. What do you mean by function well in society? Some people with down syndrome can still live a semi-normal life and be employed, are you saying your son is worse than that? Can he talk to people? Can he drive?
He still doesn't drive. He's resisted it (mostly passively) pretty hard. He even has his own car. But to him, it's an alien thing, something he doesn't want to do.
He can talk to people who share his interests at the moment, or in highly structured interactions (he works retail and is pretty good at it), but in free-form "polite conversation", he's basically paralyzed. I seriously worry about if he ever has to engage with the police, because I don't think he could obey their shouted orders.
Are you sure he's not just under-socialized and addicted to porn and video games? How does he spend his days? He might simply be a western hikikomori.