When i was 27 i was still at my parents house, buying my own food, cooking, own washing, etc, not because i was asked or expected to, but because i wanted to. This was purely because i was gagging to get out of the damn place and it was about as independent as i could get. My mum still did the ironing as she simply refused to let me not do it when i had to wear a shirt. I paid pennies in rent because i think they knew if i paid them loads then i'd take even longer to buy a property. I think that because i was quite good at saving, rarely buying toys etc. But i did wax a fair bit of money of travelling which was definitely worth it.
I wonder if your son has been pressured into thinking he isn't capable of standard life due to his illness. I know nothing about what he has, and can't imagine it's easy. I have one friend who sometimes comes out with, "such and such has this", kind of rubbish. What he still fails to take in to account is he's a property owner, has a good wife and a kid on the way and has an alright local secure job. I had another friend who is long gone, moved to Scotland now and haven't heard from him since. He was continually in and out of uni, was bipolar, and had addictive tendencies. His mum was really good to him, but long story short, he severed ties with everyone who ever tried to help him (including his mum), with his self destructive behaviour. He got some inheritance and bought a flat in Scotland.
So my opinion is, he needs some guidance and encouragement to find a chosen direction so he can work at that. I feel life is just too complicated these days in comparison to what the older folk had in youth. Far too many jobs out there now that just feel like they're only there to pay the bills and make money for the owners, to me that's a very shallow life.