Thank you for rescuing this kitty
Personally I would start with a very restrictive diet with a novelty meat. Turkey/chicken/beef can be protein allergens and unless you do an elimination diet, you won't know which, if any, is affecting his allergies.
The one food that I know of is Nature's Variety Instinct, Venison or Rabbit, canned. It doesn't contain any turkey/chicken/beef and is probably a protein your kitty may never been introduced to and is a high quality canned.
My daughter's cat does well with the prescription food
, but since it keeps her sore's away, it is better than living in misery