Make sure he has regular vet check ups to rule out anything medical. Other than that get him on a good high quality food for weight management. As far as excerise, do you have any other four legged fiends for him to play with?
Cats also enjoy interactive play with their humans. Fishing pole type-toys are my cats' favorites. They also make tiny laser toys for cats...they project a small red dot onto whatever surface you desire...many cats wil chase the laser till the cows come home (although I have seen cats that look at you like you're crazy..."Why should I chase that red dot? It's not edible!")
Light and reducing diets didn't really work for my cat. Because I was allowing the cats to free-feed, chubby learned to adjust to the lower calorie food and eat more.
I had to stop free feeding, switch to canned food and only feed 2 or 3 meals a day. He's loosing weight now on the new feeding plan.