Have you tried some different canned foods that don't have any of the ingeredients you have to avoid? Some of them have a pretty high protein and fat content. I like Merricks and Evangers canned foods. Also, I know it's probably not a permanent solution...but have you heard of NutriCal? It is a high calorie nutrirional "paste" that comes in a tube. It can be used for animals that aren't eating properly due to surgery or certain health problems.
Getting him to eat more seems like the logical solution, but that can be more complicated than it sounds.
Getting him to eat more seems like the logical solution, but that can be more complicated than it sounds.