As "that kid" in high school who was always sick, always getting new diagnoses, and overall dealing with more than everyone else, I feel your son's pain. As an "adult", though, I see your perspective more than his.
What I can say to you is don't force him. As a mother, that's probably the hardest thing anyone can tell you. Teenagers rebel--it's what they do. The more you push him, the less he'll listen. In fact, if he's anything like me, the more you push him, the less he'll do what you say JUST BECAUSE he knows it upsets you. (I'm sorry--no matter how good a kid he is, pretty much all teens have done it at some point.)
With each diagnosis I received, I needed time to grieve. I distinctly remember the time between when I knew I had a food allergy and the time I was tested for it--I purposely ate nuts because I didn't want to give them up. It took me years (and three severe reactions) before I finally stopped altogether.
One day, your son will wake up and say "gee, I really don't feel well and I have the tools to do something about it. I want to live again". I can't tell you when that will happen--it might be tomorrow, it might be years--but it will happen.
I also agree with acidrock about hobbies. I play guitar, and it was my absolute refuge for everything, especially including health isuses. What other people don't understand, music does. What I can't express because it just doesn't make sense and no one wants to hear it, music is there. It sounds like your son needs an outlet/hobby to help him along and give him something to feel better for.
Lastly, an endo or CDE who has diabetes might be of some benefit. He'd have someone who can say "I get it, I know, I'm here and I understand in a way no one else does." A person like that would also be less likely to accuse him of selling insulin (who DOES that??) as well as less likely to give up on him.
I wish you lots of luck with him and I wish him lots of luck in learning to manage. It DOES get easier, it just takes a while. (((hugs)))