Once you find out what proteins your kitty is allergic to, there are many treats out there that are protein only - so no grains, veggies or dyes, sugars etc. that he might react to. You just need to know what he can have.
If by some chance, he can have fish, try one of these brands:
wildside salmon - freeze-dried wild alaskan salmon
Grandma Lucy's brand - freeze dried Mahi Mahi or Tuna or Ocean White Fish (pollock)
Kitty Kaviar - shaved flakes of dried bonito (a kind of tuna)
Whole Life Pet Treats - freeze-dried Cod
Bella Lucy freeze dried lobster if you want to do a very special treat!
For other proteins: Rosie's Rewards - 100% beef - dried in thin jerky
Whole Life Chicken or Grandma Lucy's roasted chicken
Whole Life Liver Cubes (I believe it is beef liver)
These are just some of what is available out there.