Welcome to SR! Winnie is right, there are many avenues you may not have tried to get SSI $ for him. Many groups that are there for the disabled. I also see stores in my area that hire the disabled.
To some extent I went through a period of time that I totally took control of my AD's affairs, you see she was in a very serious car accident and suffered a brain injury. She was 19 1/2 when this happened. She was in diapers, had to re-learn how to walk, write, etc. I took care of all of her issues, car insurance, doctors appts, the whole nine yards. My thinking was that she couldn't possibly do this on her own. Today I believe that this was the beginning of my enabling. You see, she was using at the time of the accident (almost killed herself & a friend) and she did get better (because of the brain injury...alcohol & drugs affect her even more so....the doctors advised her of that)
She is now 30 yo and up to 2 months ago was still using.
It's very hard to determine if you're enabling or helping when there are the added issues. But I assure you that you paying his fines are not helping. Been there & done that. When I got to Nar-Anon I finally learned the difference. That if it is something she can do for herself, then I have to let her. That's the only way she'll grow, mature, become an adult.
You'll find much help on this site. You may also want to look into Al-Anon or Nar-Anon meetings (they saved my life)
Keeping you & your son in my prayers.
Chris