My son was overweight (22lbs at 3 months) but now he is slimming down (35lbs at 22 months).
I make sure he gets a LOT of physical activity each day. If he doesn't go to daycare I take him to the gym with me where they have a really cool play area for the kids. If you take him to places like Caesar's Palace or Chuck E Cheese for fun, he should get good exercise there. Try to get him to play so hard he sweats - every day.
Also, I do not have any junk food in the house, at all. When he's at home or with me, and wants a snack, we have bananas, cheese, or other fruit. No crackers! No Wonder Bread! If I make him a sandwich, it's on wholewheat bread (remove the crust). When I make pasta, it's wholewheat pasta.
For treats, I make smoothies. I buy frozen bags of fruit at Sam's Club, add some milk, and blend it in the blender. He loves it and it's got a lot of fiber in it.
It might be a hard transition at first to get the junk food out of the house, and he may miss a few meals. At my house, if he doesn't like what I made for dinner, than he doesn't eat. I don't go and make him mac & cheese because he doesn't like what I prepared. He's missed a couple of dinners in the past year, but amazingly, he's taken a liking to vegetables.
You might try putting cheese on vegetables at first. After awhile, he'll learn to like the vegetables.
Another thing is "dip." Kids love to dip their food. I can get my son to eat healthier things if I offer them with some sort of dip (ranch, ketchup, etc.)
Good luck!
My son like canned wax beans (Dole) and canned green beans.