I wonder OP if he has Avoidant Personality Disorder, many symptoms of which overlap with high-functioning autism.
I have had a similar situation in my extended family and I can tell you from my experience that people like this will tolerate quite high levels of privation and personal discomfort in order to avoid doing the 'normal' activities of life.
How do we think people like this managed in the distant past? Were they looked after by their parents well into adulthood? I'm sure they were not.
It is my firm belief that as a society we have taken away so many of the lower-level jobs and activities that could enable people like OP's DS to have self-esteem, purpose and independence.
In my experience there is often a snobbery within the family that their otherwise-disengaged adult DC is somehow 'too good' or too clever to wash pots, or push trolleys, or do a very basic factory job, or do cleaning or security work.
That's just not true. Anyone who can play dungeons and dragons, go to the gym, and complete employment support programmes can do these sorts of jobs. Whether they can stick with them long-term, or progress to something else, or ever manage anything vaguely high-flying is a different question, but an unnecessary one for now.
But in the course of the whole of human existence, being able to please yourself while you have a roof over your head and are being looked after is an indulgence, or otherwise something provided only for her most needy.
I'd say start modestly with him in the jobs market OP. He could do agency work, zero hours stuff, anything, he needs a long-term timescale to become independent.
Give him a year. Give him shorter deadlines in between to register with agencies, do regular volunteering or whatever. If he has grandparents n then he should routinely be going to help them with shopping, small jobs around the house, or companionship at least.
Make him get out and go for walks every day, and contribute to the household activities.
Measure him always by the effort he is making, not necessarily the outcome he is achieving.