I think maybe when things calm down have your son talk to the boy. He can tell him that he can't help that he has diabetes, that guess what *IT SUCKS* and he wishes he didn't have it and didn't have to stop because of it. But, he DID have to stop BECAUSE he has diabetes and that he was more hurt that his friend didn't stick to the plan to stay together. Carson can even say something to the effect of "I would have stopped if you had a cramp, because I would have known you couldn't help that either" and see if they want to give it another chance. Having Carson acknowledge to his friend that it sucks for him, maybe friend will realize and have a little sympathy and realize the reality and they can both move on.