Have you calculated how many calories he should get in a day? A common formula is 20 calories per pound of healthy body weight. You could adjust the amount you feed him in a day to make sure it is within the calories he should have. I mix chicken bone broth and water into my girl's food. Maybe you could do something like that to add to the volume.
There are a lot of wet foods that are lower in calories than Fancy Feast. I feed my girls Weruva Chicken Frick a Zee which has 76 calories in a 3.2 oz can. (Fancy Feast Chicken Feast has 99 calories in a 3 oz can.) Tiki Cat also has lower calorie foods.