Nut Butters to the rescue! If he is having a problem with animal fats he might still be okay with plant based fats, which are more natural. Nut butters are high in protein, the best kinds of fat, and many minerals. Almonds are very rarely allergenic, unlike peanuts. I like Sunbutter which is from sunflower seeds (found at Target) and Almond butter. Mix these in a blender with some original flavor rice milk and a splash of virgin coconut oil (which is a medium-chain fat, the kind found in breast milk) and this will go a long way to increase his caloric needs. You could add a little bit of vanilla to give it a nice smell.
Kool Aid has lots of food coloring, which should be avoided because its not good for anyone. If you want to fry stuff try frying in olive oil and see if that makes it better. If he will eat avacado that is also one of the healthiest foods out there.
I hope you keep pushing your doctors until you find some help that can get your kiddo well. It sounds like you are still alone in the dark and not sure what to do. You are his best advocate, be strong and dont be intimidated by doctors.