How can I confront my boyfriend about his eating habits?

My boyfriend is a great guy, but heats an extremely unhealthy diet. This bothers me because I'm a very health conscious person, and I work hard to stay in shape and take good care of myself. Not only am I concerned about his health, but his attitude towards the whole situation frustrates me. Every time he tells me what he had for a meal it's chicken nuggets, corn dogs, pizza. And then he'll tell me about his stockpile of chocolate bars to eat while he's working on his essay, or how he's going to the store to get discount Halloween candy. He does eat some fruits and vegetables. But you can't make up for a generally crappy diet by eating a carrot here and there, right?

The other issue I have with the whole thing is that he has gained some weight since we started dating. This is NOT a big deal to me- except that HE frequently complains about it. It puts me in an awkward position because he'll say, "Ugh, I'm getting a belly" and I'm thinking "Duh- you just ate five candy bars." Of course I reassure him that he's attractive to me, but I hate how oblivious he is about the whole thing. He takes a yoga class, but it's not enough to counteract all the junk.

I've tried to gently comment about his diet, but he'll tell me "Don't worry about it" or "It'll be fine". He thinks he can eat whatever he wants with no consequences just because he's young. But this is something that bothers me more and more by the day. How can I express my feelings without being critical or making him feel bad?

Obviously it is ultimately his choice. I don't want him to feel like I'm controlling him or forcing him to do anything. But it would be beneficial for him to eat better for his own sake. And I can't live with this forever. It may seem like I'm making a big deal out of nothing, but it's an extreme situation.
How can I confront my boyfriend about his eating habits?
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