He's probably overeating. You're not supposed to feed a cat more than twice a day. My friend's cat did that a few times when it overrate. Once it figured out that secretly eating food = bad feeling it stopped doing it. Feed your cat no more than twice a day. Once in the morning. And again 9 or so hours later.
Some cats can't seem to control their appetite. Back in PA, one of mine would only eat until he was full, and the other would just eat everything you put down, so we had to watch him. He also weighs like 25 pounds.