Is he 15 pounds or 22 pounds? That's a huge range for a cat.
What is his normal food?
Stop ALL treats. He should have absolutely nothing other than his normal food.
I would put him on a high quality dry reduced calorie diet in controlled portions. Acana makes several formulas in "indoor" cat varieties. Read the recommendations for how much to feed a cat at the target weight your cat should weigh and feed him no more than that amount daily. Try giving it to him in something to make him work for it.
Like this.
Taking him for walks on a harness would be good if he'll do it. Most cats resist that.