Sounds like a typical teenager indeed...
It's is difficult to stand on one side and watch, but there is little that one can really do apart from wait until he comes to his senses..
It might be a good idea to give his DSN a phone, let them know what is happening with him and what he's doing etc... Hopefully they can explore this at his next appointment, you might even be able to swing a eariler appointment for him..
It sounds as though in many ways he hasn't as yet really come to terms with his condition and what it all entails on a daily bases, he seems to understand his need of insulin, as you say he does take it at select points, and I suspect that he doing this because he actually knows it makes him feel better when he feeling pretty rotten with his high blood sugars...
It may be that he needs a bit of help to really come to terms with his condition, and being an all male teenager (who happens to know best etc like they all do) he's actually finding this difficult..
As I said, ring his DSN have an heart to heart, not only about what your son is upto, but also how frustrated and rotten you feel in yourself about not being allowed to help him (his choice not yours or theres) they might be able to come up with some suggestion...