Hi and welcome to PT. Since you've already stopped free feeding him, I would suggest trying a different kind of cat food. Many of them have a lot of carbs in them and very little protein which is what cats need.

I currently feed all of my cats Innova Evo which is a grain free cat food and you can buy dry and canned food in this brand. I don't free feed either and 5 of my 6 cats have lost weight while eating this food because it's a very high protein diet. It has a lot of calories too so they don't need to eat as much of it as regular cat foods. My 6th cat is on prednisolone which has caused him to gain weight due to drinking a lot of water. You can buy this brand at most independent pet stores.

Another great way to help your cat get more excercise is to buy him a laser light. You just sit on the couch and let him chase the laser all around the room. Most cats love it. Good luck.