First, stop buying junk food. There are plenty of 100 calorie snack packs out there, plus popcorn, fruits and veggies. If he drinks a lot of soda, maybe switch to diet or ice tea or natural fruit juice like V8 Splash. (Although you still want to limit the juice due to the amount of sugars, but it is a healthy alternative.)
What would really help is if you both do something. I don't know if you feel the need to lose a few pounds, but maybe if you bring it up as something both of you can do, he will be more motivated to have a healthier lifestyle. I highly recommend Weight Watchers, because it is not a diet. It's a healthy lifestyle change that forces you to make healthy choices and decisions. Their web site has toms of great recipes. There are other sites too like the South Beach diet and flatbellydiet.com.
Then maybe you can make time to take walks together or go to the gym together.
The thing I love about Weight Watchers is that I do not even need to exercise and I still am losing weight.
You definitely want to sit down and talk to him about this. Tell him that you love him and you are concerned about his health. Since he feels self conscious about it already then I am sure he would love to feel healthier. If you let him know that he can get healthier without depriving himself, that may make him feel better too! (Another WW perk! I don't need to deprive myself!)
Portion control and a balanced diet is key in losing weight. 3 helpings of a healthy meal is not going to do it!
Although if he is not receptive to this now, just continue to support him and buy healthy foods. (Try and only make enough so that there are seconds and no leftovers.) Don't comment on what he eats or how much because that will probably just backfire and make him eat more. (I'm speaking from experience on that!) Instead if he complains about his weight at all, let him know that you support him but that he needs to make the change and the decision to lose the weight.
I struggled with my weight gain fro several years and my husband never said anything unless I complained. Then he would say "well, do something about it or stop complaining." It may sound harsh but he was completely right! NOw that I've finally done something about it, I feel great!!
I hope you can find the right technique and plan that works for both of you! I'm sure he will come around but he may have to do it in his own time!