When was his last deworming?
How is his appetite?
How is his energy level?
Is he an indoor or outdoor cat?
It definitely sounds like he is sick. A possibility may be that he has intestinal parasites, or worms, that is causing his stomach to distend. Other options that make a belly enlarge would be retaining fluid from a heart problem or a mass or enlarged organ, both situations are very seriousl If he isn't eating well and has low energy - this is a sign he is sick. I think you should get in to see your vet and let them determine what the problem is before he gets much worse.
If you can, collect a stool sample to take with you so they can check for parasites.
Let me know if you have more questions,