Just gotta stick to a strict feeding schedule. Find out how many calories are in your cat food and limit his calorie intake. On an episode of The Dog Whisperer he helps a dog lose weight by putting him and his portion of food in an enclosed area (like the bathroom) for 10 minutes or until he finishes the portion. If they don't finish it empty the bowl. Keep doing this at every feeding and eventually they won't need to be enclosed and they'll eat their entire portion instead of taking a bite and then coming back later to eat more (like my cat does). This allows you to be able to control their intake better.
"Being overweight has a drastic effect on your cat's health and happiness. If your cat needs to lose weight, restrict calories by 10 percent or less. It will also help if you feed your cat more mini-meals, rather than one or two big meals. You may also want to try reduced-calorie cat food, since it will enable you to feed your cat the same volume of food while still lowering calories. Cats should lose no more than half a pound per month."