Since the brands and dry food aren't working well, I think it would be a good idea to try something else. Usually, if a cat has allergies/sensitivities to certain food, (e.g.chicken), the usual advice is to try the cat on a novel protein
, something he hasn't had before, e.g. lamb, rabbit, kangaroo meat. Usually the best food is one that is natural, without additives, such as berries, veggies, grains (rice, wheat, soy, etc.) as cats are "obligate carnivores" (in the wild they only eat meat, and maybe a little grass). There are some different types of food that are meat that is freeze-dried pellets that are softened with water.....you might want to try one of these in a novel protein. Some pet stores carry frozen meats of different kinds. Some people make their own cat food. In the following link, there is a recipe for making your own cat food. So hope some of these options are helpful. All the best!
Here's a good link of proper nutrition for cats: https://catinfo.org/