Relationship Woes

Ok, so I sort of have a problem. I typically eat pretty healthfully...low in calories, low in fat -- and of course diet sodas. However, my boyfriend does not share in this appreciation of healthful foods. Day after day he eats junk food, pizzas, deli sandwiches loaded with high calorie condiments, and the list goes on. Luckily, I have turned him onto diet soda instead of some higher calorie alternatives but I feel like that doesn't even make a dent! I don't want to be rude or controlling by trying to get him to eat more healthfully, but it is really getting out of control. And he just eats SO much, more in one sitting than I eat all day! Seriously. I'm more concerned about his health than anything. Does anyone have any tips of how to approach this situation without hurting his feelings or seeming like a nag?