Is he insured? If so it might be worth getting your vet to take a blood sample from him and getting it sent off to York for allergy testing as then if he does have any intolerances/allergies you'd know exactly what they were and how severe they are. You get a full report back of everything they tested for allergies and intolerances to (IgE scores indicate a true food allergy and IgG scores indicate an intolerance) and a score of between 0 and 5 to how allergic/intolerant to the substance the animal is.
The full allergy panel tests for all sorts of tree, shrub, weeds and grass pollens, human dander, dust mites, storage mites, moulds, fleas and various meats, fishes and other food stuffs like milk, wheat, maize (corn), soybean etc
Also if he has allergies confirmed most insurance companies will contribute towards the cost of prescription food.
The Royal Canin Duck and Rice pouches and trays are also maize free as well as wheat free and JWB is wheat free but full of maize so if you've got a cat who's allergic to maize they'd still be allergic to most of the hypoallergenic foods you can buy in the shops (JWB, PAH own etc)
. Royal Canin do a chicken and rice flavour in the Sensitivity control which is also maize and wheat free.
Arden Grange Fish and Potato Sensitive is a good hypoallergenic dry food (and cereal free in case he does have a wheat or maize allergy)