Depending on what diet he's on, see if they have a canned version you can feed instead, try tricks such as:
putting the food in the basement/or upstairs bathroom so a flight of stairs has to be gone up or down, to reach the food.
schedule a play session daily using an interactive toy such as a wand toy to get your kitty running and chasing, and provide self-play toys that encourage movement such as various ball cats toys that will roll.
If you do stay with a dry food, and have been free-feeding, ask your vet if you can switch to feeding 2-3x a day, and simply divide the recommended amount of food (based on your cats weight, there will be a recommended feeding guideline on the package) into 2 or 3 feedings, and consider feeding sl. less than what is recommended.
It is important that a weight loss program be done gradually, and having your vet supervise this is best.