what do you feed a cat with bad allergies

[Dukkha]

I have a cat with allergy problems. This has been going on for a couple years now but recently has been getting worse. He sneezes a lot now, his weight is going down, and he vomits more frequently. He's been to the vet three times over the years and they never help him, because he doesn't display symptoms in the clinic. He will never, ever take pills or any kind of meds orally. He sees me with a Benadryl bottle and runs & hides. My only thought is to change his diet. Regularly he got 9-Lives canned and Purina ONE Urinary Tract Health Formula dry, because he had Urinary Tract problems before.

So is there any formula food specific for cats with allergies, or a good bland diet that is easily digestible for cats?