I don't think there are really diet pills that work, and even if there were, I wouldn't give them to a cat.

You should probably switch to low fat food, and try to get your cat to do some exercise. Try putting the food in a different place every time, and in higher places, so that he has to "hunt" the food. Limit your cat to one brand, and only dry food (it can be premium, but no treats). If the food is less "interesting" the  cat will eat less.