OK. First of all assure him that the clinic have heard every excuse in the book, and are there TO HELP HIM. He won't get into trouble, and what he needs is their support. If your hospital clinic isn't like this, then find one that is. I believe paediatric diabetics are entitled to psychological support via their hospital clinic.
Secondly, you say he is eating things he shouldn't. What shouldn't he be eating? Nothing as far as I know, so long as he is dosing his insulin accordingly. He needs to carb count, then he can eat whatever he likes. He's a teenager.
Thirdly, ask about a DAFNE course it will help you and him learn how to adapt doses to accommodate exercise and eating different meals everyday and also missing meals if he wants.
Fourthly, if you are frightened he won't comply with his regime, and he has only been at it a couple of months, then perhaps, at some point get him to see what can happen when you don't take care of yourself. The most frightening complication that can affect a boy is impotence. A friend of mine had type 1 from around the same age as me (early teens), he smoked, didn't go to clinic, didn't test etc, and died in his mid 30s. And it took him a couple of years to die too. I on the other hand have lived another 10 years than him and am in good health.
You are his mother, it comes with the territory to worry yourself silly about this. If I were you, I would too. However, he is 14, and still wants to be the boy he was a few months ago. He is still "Joe Bloggs", he doesn't want to be "the diabetic", and he doesn't want to be different. Don't appear to him to be watching his diabetes over his shoulder, teenagers rebel, and you don't want him to use his diabetes as the rebellion. It's a fine balance between ensuring he does everything right and turning him in on himself. Sorry if I sound harsh, I don't mean to, I remember being a teenager and not wanting people to see the diabetes first and me second.
Good luck.