Have you ever been on a diet (or not) and had someone telling you constantly that you are doing it wrong. Even when you were doing it wrong? If not, I have, and it is something I rightly or wrongly resented.
The crappy thing for you is from the 'helper' side, you are pretty much in a no win situation. You feel you just must 'help', but any help will be interpreted as nagging.
Oh, and it get's crappier: At the stage he is in it's almost impossible not to lose weight. He is learning that he can do what he wants and lose weight. In a few months he will learn this is not true and then it will be too late to build success with weight loss as a positive feedback.
Really, you have a bunch of bad choices to choose from.
I have a relationship that we can sit and talk about these sort of things, but then that's it... it's up to us do to it. Frequent reminders do not go well, we work at supporting the right things, but don't force or push. It's an almost impossible balance that we often don't manage.
Good luck,
Tek