Often there are seasonal allergies besides food allergies. Allergens are peaking right now. DERMA-IONX is helpful for a variety of skin conditions. It is an oral homeopathic product which stimulates the enzyme systems in the body that aids the skin and helps with assimilation of nutritional components in diet and supplements. The supplements I use along with it include Probiotics and Omega 3,6,9. The OMEGA 3.6.9 is especially helpful for soothing the skin. These supplements together with DERMA-IONX can produce a synergistic response. I would give a half dose on the breakfast and supper food. After there are signs of improvement you can give a whole dose once a day. I would also suggest that you give your cat only filtered water for drinking and bathing if your water contains chlorine or fluoride.
For the dietary modifications you might want to try a grain free diet and feed cooling foods according to Chinese food therapy. If your cat is warm and seeks cool places to relax in he might do better with the cooling diet. Fish and turkey are considered to be cooling so would be better choices than chicken or lamb which are warming. Of course if your cat has a food sensitivity to a particular food do not give that food. (Beef is neutral.) What kind of special food have you been giving? The hypoallergenic foods are not always the right fit.
Another thing that is important for cats is to have stool samples checked regularly. Tapeworms make skin symptoms worse. Looking forward to your reply.