Talking to someone about their weight is SUCH a touchy subject. I'm sure you already know this, but you MUST tread lightly with this discussion, or the situation might get worse...
Don't tell him that his belly is getting in the way of your sex life, or that it grosses you out to watch him wolf down food.. it may be how you feel, but you're going to hurt him. I know you said that you've tried to explain nicely before with no results, but getting this direct will not help either.
Have you tried swaying the conversation that YOU want to get healthier, to try to coax him into a healthier lifestyle too? It is much easier to become a healthier person when you've got a "partner" to do it with you and help support you. Maybe try telling him that you want to change some things in your lifestyle and you want him to do it with you. Start cooking those healthy meals, and ONLY make enough for each person to have one serving. He can't go back and eat 8x's the portion size if there is only enough for 1. Throw out any/all junk food, sodas, etc in the house and replace them with only healthy snacks so if he tries to get around the smaller meals by eating junk, he can't. Start on a moderate exercise routine and ask him to join you... even if it is just a brisk walk before or after dinner... or join a couples sports league... or challenge each other on the Wii or similar if you have it.
I did this myself over the summer, only I really was just trying to get myself healthier, but because my lifestyle choices impacted my boyfriend, he also ended up losing about 20 llbs. And that was just by me changing the food available around the house, and taking the dog for longer walks.