It's natural for adolescents to test the limits of their behavior in order to assume control over their lives. What's more, teens tend to think only in the present tense and believe that nothing could ever possibly hurt their young, strong bodies. Although these are normal, usually harmless phases for most kids, such delusions of invincibility could be dangerous for your diabetic son. The challenge you face is convincing your teen that managing his diabetes now will keep him healthy down the road.
Since many adolescents are averse to parental guidance, it's important that your son has a close, open relationship with the doctor managing his diabetes (this may mean changing practices now that he's older). After all, he's probably more likely to follow the advice of a respected medical professional than that of his nagging Mom and Dad. Before his next scheduled visit, give the physician a call and discuss what's been going on. This way he can broach the subject with your son (it might be best if you stayed in the waiting room), talking about all the risks involved in not managing diabetes properly and coming up with a treatment plan that works better for his busy lifestyle. Maybe the glucose testing and insulin injections he's doing are interrupting team practices, for example. There are many different ways to handle diabetes, so your doctor should be able to come up with something your son will agree to follow. It might also be helpful for your son to connect with other diabetics to see how they manage their illness and to have someone to commiserate with about what a pain diabetes can be. Ideally this person will be a relative or family friend your son already knows, but a support group for diabetic teens could work just as well.
In the end, letting go and allowing your son to be responsible for his diabetes (as scary as this may be for you) might be just the vote of confidence he needs to rise to the occasion and take control of his health.