Veterinary nutritionists will consult with you by phone.
You can google it and some will come up.
It is important to know the percent of fat that he is getting and they can help you with that. He may need high fiber also. Fish may be a good protein or possibly very lean chicken. I would put him on an extremely low fat prescription diet until you talk to a nutritionist because you will likely need exact amounts of ingredients if you homecook that only a nutritionist can help you with.