I've got a slightly different perspective due to my experience.
I have mental illness and so does my one biological son. Neither of us were totally able to be independent at 25 due to this, although neither of us took drugs. My son has severe Obsessive Compulsive Disorder (OCD) and had to drop out of college because he was both too filled with anxiety to go and unable to stop counting the words of his professors. So he couldn't understand what they were saying. He was on disability for three years, but he got some GREAT psychiatric help and was able to at least get a good job and marry. His father, my ex, was willing to fund his psychiatrist and medications as long as he tried hard to get well. He did and nobody regrets helping him, although he was at least 22 before he was able to get off of disability. Now he has actually moved away from his family for a better job (he is now 31). He wouldn't have been able to move away from Chicago before. As for the job...he is making as much as a college graduate now....his psychiatric help was a godsend in every way.
As for me, how I dealt with being mentally ill, I got married at 20 hoping to be taken care of because I knew I couldn't take care of myself. I didn't dare ask my parents for anything...they wouldn't have given me the time of day, but would have hung up the phone. The marriage wasn't good, but I had a place to come home to and I did have kids. It was better than the streets. With tons of therapy, self-help, and trying very hard I have really come a long way. If I needed to be independent now, I could be. But it took a long time.
Here are my thoughts about your situation: I don't think you need to support you son, but he sounds like he has some mental health issues that are quite serious and could be impeding him in his life. If I were you I'd become a broken record and tell him to get psychiatric help. If he complains that he has no money, tell him to do two things: First he needs to go to the local mental health clinic. I do, and I *love* my therapist. He can also apply for social security. If you want to help him pay for BETTER psychiatric services, I think that is appropriate as well, as long as he TRIES to get better, respects you, and you do not help him in EVERY way.
The bottom line is, whatever you decide to do, be very firm and stick to your guns because if you give an inch, he'll take a mile. It does sound to me that he is quite depressed and his friendlessness is worrisome. Can he at least talk to his stepbrother?