If he has acted that he's not really into it, and you can't talk about it, then he's probably not into it. So, you can either enjoy yourself and hope he doesn't diet, get the courage to talk to him about it, or find someone else.
Seems like every day someone posts this question, so I'm sure you'll find some thoughts already posted in the forums. But ultimately it comes down to what johnxyz just said. If you don't ask, you won't know.