Cat allergic to chicken, beef, lamb, Turkey, pork, white potato, and rice. What in the world can I feed him?

Adopted my sweet boys almost two years ago. One(who was the runt of the pack) has had nonstop cough attacks ever since he was a kitten so we finally got him allergy tested. He came back allergic to basically every protein under the sun(which is coincidentally all his current diet consist of). He’s completely fine with seafood but I worry about feeding him solely seafood because of the mercury levels so I want to include duck, or rabbit or venison as well. Only issue is EVERYTHING is so expensive. Anybody else have cats with similar allergies? What do you feed them? Is there any way to keep costs to a minimum?

(Currently my boys eat mainly wet food and freeze dried pellets. The one with allergies is incredibly picky on top of everything)

EDIT to add more info: little guy received Blood allergy tests through Nextmune. His current diet consist of a variety of chicken, Turkey, herring, whitefish, salmon ane lobster and only eats wet and freeze dried as treats. He has no skin conditions or digestive issues and has been cleared for asthma but has frequent asthma like cough attacks.