You are great people to be doing this for this kid.
The "legal" questions...ie-truck, Social Security number, credit reports, bank accounts...I'd probably talk to an attorney or go to each individual agency. For the visit home to pick up his things...call the police and ask them. For the truck, call Motor Vehicle or the police and ask them about getting the father's name off. At the very least, can you get the locks changed on the truck so that "Dad" can't come and take it one day, crash it, and cause this kid all kinds of grief with insurance etc. Credit reports...talk to the bank or a credit card company and see if some kind of "warning" can be attached to his name and SS# to not allow accounts to be opened. I'd talk to SS about any SS questions. Bank account...again, talk to the bank. Realistically, if they won't take the mother's name off the account, and it's an "either/or" account, then I'd close it or at the very least get his name off it. Don't let her overdraw it or something and screw up his name/credit...whatever. Write everything down and get the names and phone #s of people you talk to. Also jot down the dates and times when you speak to people. Meticulous record-keeping is key here. You don't want to make a misstep and give the "parents" (and I use the term loosely!

) any kind of power or legal ability to harass this kid.
I can help you with the medical stuff.
Since he probably doesn't want his parents making health care decisions for him, the PP who suggested a Healthcare Proxy was spot on. You need to do that ASAP, since with his diabetes, his condition can change in a heartbeat. VERY, VERY important!!!!!!!
There is a lot of help for diabetics as far as their medical supplies go, at least in my area. Tell him to speak to his doctor...many of the pharmaceutical companies that specialize in diabetic products have programs for people in his situation, who may not be able to afford supplies. Tell him to talk to his doctor about samples. Call your local hospitals and ask them if they have a diabetic nurse or any kind of diabetic program where he might be able to get supplies. Call the American Diabetes Association...they may have some leads too.
IIRC, you said that this kid's family likes to keep up "appearances". There's a little evil side to me that would like nothing better than to have their whole neighborhood see the kid have to be escorted to his house by the police to clear out his things and protect him from his parents.