He may have a reaction to the brand of cat food you are using.
Cats can be sensitive to corn and soy products, particularly. Look for a brand that has meat and rice as the first ingredients. If you are feeding canned food, try to stay away from fish. Try turkey or chicken instead.
Any changes should be made slowly as a quick change can cause more gastric irritation. Mix 1/2 new brand with 1/2 old brand of food for the first week before switching over completely to the new brand.
Also make sure that the cat isn’t eating too quickly which is allowing air into the body. You can do this by raising the food bowl so it is off the ground by a few inches. Also feed the cat two or three times a day instead of once a day.