I have a couple of guys that have this issue, so I understand how frustrating this can be. (And yes my guys are on rice and chicken) But I also add some canned pumpkin and that usually takes care of things, with the side benefit of reducing hair balls in everyone

If he won't eat the pumpkin, you can always try a baked sweet potato or yam (which also does the trick).

You might need to keep him on a low-dose prednisolone to keep things under control. (It looks like I will have to do that for one of my guys ...)