The most common reason for the situation as you describe it is eating too fast

If the cat is eating too fast you need to slow down the eating. You can put out the food a tablespoon at a time. You can divide up the meal into clean tuna-sized cans he has to push around the floor and clean out. You can put whole walnuts-in-the-shell in the food dish and he'll have to eat around them. I'm sure you can think of other ideas.