My boyfriend is obese, I'm so worried about him--help!?

My boyfriend is obese. He is 70 pounds overweight. He was really heavy when I met him last year, but the weight is now out of control (he has a LOT of stress at work). His doctor has told him he MUST lose weight and how much he needs to lose, so I know this for a fact. Plus, he's gaining a little bit every week. Plus he smokes heavily, is nearly 40, and lives on a diet of junk food. In every other area of his life he is the best man I have ever met, and I love him dearly--so much so that I don't want to lose him at a young age. He's already sleeping on his couch because he gets winded going upstairs to his room. I've tried to get him to eat healthily, but I've discovered a.) nagging does not help at all, b.) even gentle reminders are nagging if you don't want to hear it, (I mean, I don't like to be nagged either--no one does), and c.) even though he says he wants to lose weight, when he asks me to help him he then starts hiding fast food meals (eating them in the car, at work, and so on) and then lying about it later. He also fudges big time on what exactly he weighs when asked. The doctor will ask him for his current weight, and my boyfriend will lowball his estimate by at least 10 pounds, and then when the doctor weighs him because honestly my boyfriend is visibly larger, he is shocked by the real number. He has a really warped relationship with food, it seems. It's a reward for a difficult time (dental surgery = two coneys heavy all), and a bandage for a bad day at work. How can I get him to eat better so he doesn't drop dead of a heart attack? (We'll leave the smoking issue for a while)