Hi Alex! Do you remember which food he seemed to do better on? All of the foods listed have potato listed except Zignature. Have you tried any red meat sources? Like bison, lamb or venison? Are there any ingredients that you know he can’t have? That would help a lot figuring out what food to try.
I don’t buy into the whole prescription diets thing. I don’t really see anything in them that you can’t find in non-prescription foods. You just need to know what is “special” about the food and find one that meets the special requirements.
