What should i do to help my boyfriend stop overeating?

You can help a lot.: Help him feel good about himself, and about how much better he'll feel if he takes great care of himself. If mood/anxiety are issues, treatment with meds can be helpful. If lack of success is a factor, seek practical self-improvement steps +/- therapy. Most important: learn to prepare fresh, healthy, tasty meals with moderate portions - make this fun for both of you -- gourmet cooking is "in" now.