Nothing will make a guy angry like telling him no more steak, ever, or snatching a cookie out of his hand. Even if he's getting tubby, he's going to resent you for telling him he needs to change his diet.
Men eat a lot of junk; there's no avoiding it. Before we were married, I could count the number of times I made myself a salad on my thumbs. But now I eat one four or five times a week simply because of my wife's persistence. At first, she made it seems like a simple offer: "Can I make you a salad?" Sometimes I said no, but after a while I started saying yes every time because it was easier. She just wore me down. And then all those fresh vegetables she brought home from the market started showing up as side dishes. Now carrots and snap peas materialize in my lunch whenever she packs it, and I don't mind.
If you want a guy to change his diet, don't put it all on him. If you slip him something healthy on the nights you cook, don't deny him that slab of ribs or extra piece of pizza when you go out for dinner. The best place to make a change is at home, because both of you will still eat most of your meals there.
Get physical together