Benman2002 has a pretty good idea there.
Watch a movie like An Officer and A Gentleman or something, and then tell him "you know, you'd look great in a uniform" Boys love going macho-style, and the notion of a uniform would no doubt appeal to him, no matter what he says (cause he might try to hide his boyish enthusiasm).
Most important, if he's insecure, you have to make him feel secure. You know, he has to be certain you love him no matter what, right? For better or worse.
Getting in shape is something people do to make themselves feel better, so he's gotta want it so bad that he gets up an hour before work to do his cardio, and when he get's home the first thing he hits are those weights.
If you want to talk to him about it in way that doesn't arouse resentment, so that he himself will realize it without you having to bash it into his head (which by the way, will only make him more ill-willed), may I recommend a GRRRRRRREAT book.
It's called "How to Win Friends and Influence People" I know it sounds like a book for rejects and business men and politicians bent on manipulation, but this book is all about being sincere, and genuinely benevolent to get you the results you seek.
The author is named Dale Carnegie. The book was written over 80 years ago, but it's no less important or correct.
http://www.amazon.com/exec/obidos/tg...71968?v=glance