This happens to so many of us - but please find comfort in the fact that many before him have gone thorugh something similar and come out the other side just fine.
I don't know that there is any magical answer to how to "fix" it. It was a combination of things that helped me become motivated again - support from other T1's, finding the diabetes online community (DOC) and reading other people's stories and blogs, and getting equipment that better fit my needs (insulin pump, CGM). Does he have anyone his age who has type 1 that he can talk to or hang out with? It's hard at that age for anyone, but when you throw type 1 into the mix too, it can be a lot to handle on your own. Does he use this site at all?
It's hard to watch someone you love not do everything they can to take care of themselves. Nothing is easy about it. Just be there for him as much as you can. Hugs help, too.
I write a lot about the struggle I've had with acceptance and motivation, as they relate to my diabetes care, on my blog. Like I said, sometimes even reading about someone else who is or has gone thorugh the same things can help a person feel a bit better. If you think it will help, send him the link I have at the bottom of my posts.
Good luck to you both, and please keep us posted.