Get him to go to the doctor and have the doctor tell him how much danger his health is in. Make the appointment FOR him!! If you are the type to cook dinners all the time then cook healthy, even if he doesn't like it at first. Lean chicken breast, salads with tasty dressings. If you don't cook then pick healthy restaurant and keep ONLY healthy snacks (dried and chopped fresh fruit, nuts, yogurt, even low-fat beef jerky) around the house. REMEMBER: ITS NOT ABOUT LOSING WEIGHT OR A "DIET" IT IS ABOUT BEING HEALTHY AND HAVING A HEALTHY LIFESTYLE!!!
The other important thing is to start developing active hobbies, like walks and tennis or something like that. Something you can do together, or by yourself-- set a good example and ask him to join you! Talk to him about spending more time together doing active things--not everything has to be a workout, you know! Does he like basketball or other sports?
Tell him how much you love him and how you want to spend a long and healthy life together!
And if that doesn't work, you could always withold sex until he loses a certain amount. (Five pounds the first week gets you a bj, 15 pounds by the end of the month gets two hours in the bed with lingerie! LOL, I'm just kidding!!! :)