My heart really goes out to you. So many things i want to say to you but i'll try not to overwhelm you to start with. The medically fragile child is such a different situation and so very hard on a mother. My son is diabetic and i spent so many years caring for his body and catering to him that i kind of created a monster - very needy, very selfish, and very expectant of me fixing everything for him. Your sons disability I can only imagine becuase he cant fall like some others can. I understand this - it is different when you have a child that cant care for themselves but maybe you can find him the help to learn to care for himself.
A couple of suggestions i have for you.
One test means nothing - this i would fight for because he needs this help and it may be the only way you can stop being responsible for him. If you working with the county level - move up to the state. If you are in the state level move up to federal. Try
Substance Abuse and Mental Health Services Administration (SAMHSA) Website. Maybe he can at least get a partial disability. My son required help from a facility that could handle the substance abuse and could handle the medical needs - this is just non-existent if you dont have a fortune so i jumped through the hoops, kept moving up the chain of command and finally the state and federal started putting pressure on the local level. Everyone wanted to get rid of me but they didnt get rid of me until they gave my son the help that he has just as much right to as anyone else.
Even with a disability he can get a job - I know that Kroger in my area hires a lot of mentally disabled people. This could be a way he can work his recovery and learn to start supporting himself.
Talk to his PO - if he is open they may be able to open up a range of services for your son other than just jail. not all POs are going to be helpful but you may get lucky. If that doesnt work go to your local mental health department - push them to test and retest, find out other programs he can get into.
A lot of churchs also have programs to help disabled and you may find that they dont have the same requirements. I saw several church programs for this in my area. They may be able to help him not only with addiction but also some training that he is going to need.
Also call the United Way - you can just dial 211 and they may be able to help as well.
The hard part for you is detaching the two issues. You have an adult child that is not a capable adult and you have a child who is an addict. Even with a disabled child you can find boundaries - you can have limitations. Its going to be harder to figure these out but he's going to be better off once you do. With his mental capabilities it may be easier for you to think of him as a younger child with this problem.