I wrote a post like this myself a few years ago. Over the last year my dh has lost 60 pounds. The only thing I can really say, is that he will do it when he is ready. I'm sure you know this but you can't nag, bully or convince someone to take charge of their health until they are ready to do it.
For my dh he was well over 370 pounds and the thought of taking on such a huge obstacle was so overwhelming. He just didn't know where to start. He finally realized that he didn't need a "diet" he needed a lifestyle change. He stopped eating all fast food, except Subway. No pop, no candy, basically no garbage. Fruits and veggies. There were many times where he cheated, but once he got used to it he stopped wanting it. He's also learned that you can have one piece of candy instead of 20. Portion control. His next goal is to start looking at making sure he's balancing his diet.
It's amazing what losing 60 pounds has done for him. He bought a bike(a big big thing for a guy who really just likes to watch tv) and wants to play baseball next summer. He has more energy, he sleeps so much better, he doesn't snore and is willing to do things like go to the zoo again. He used to get tired and his back would hurt from walking, it still does occasionally, but it's so much better. I can't tell you what it's done for our sex life either
There's a Dr. Phil weightloss book out there that has a alot of really interesting stuff in it. Though I despise Dr. Phil, the book gives a lot of different ways to figure what your food triggers are and gives a lot of great ideas on how to change your food habits.
If your dh is interested I know my dh would happily offer advice. Sometimes talking to someone who has been there (or is there in my dh's case) can really help with perspective.