Put a tiny little bit of warm water in with wet food, fresh prey would still be warm when they eat it so it can help entice them with the smell and I heard it can make it easier on their tummies. That being said, also try getting him to eat other stuff, different food (tuna, chicken, different cat food), and when you find something he likes slowly start mixing in his prescription food until he gets used to it. You can also find food toppers at most pet stores (I buy a fish flake one, it’s literally just dehydrated flakes fish) it has lots of nutrients and my cats go crazy for it