another way to takle this would be to send in some blood and let a lab determine what your cat is allergic against.
Here's a German lab that offers this test:http://www.laboklin.de/frame.php?lang=en
(look under products -> allergy)
Screening test for seasonal and perennial allergen groups, dog/ cat: additionally flea saliva
Allergy main tests : differentiation of seasonal and perennial allergen panels, insect allergen
Food allergy test: dog / cat
IgE- and IgG-antibody panels for 18 different allergens
or try out an elimination diet as sharky suggested by sticking to one type of meat source for 6- 8 weeks. This meat source should be uncommon in average cat food, so I personally wouldn't recomend duck or chicken as your cat's allergy could be against these meat sources. Pure horse meat would be better, Boos is a cat food brand containing pure mustang meat sold for this purpose (it doesn't contain Taurin either, you'd have to add this if feeding it long term). Venison or any other exotic meat source would also be OK, such as venison, ostrich, kangaroo ... but these are normally much more expensive and may be difficult to obtain. Also I wouldn't feed any treats what so ever, as these may trigger the allergy.
If your cat is OK after 6-8 weeks you can then add a new meat source and try this out for another 6 weeks, and so on ... slowly adding to your list of foods that are OK or not OK for your cat.http://www.zooplus.co.uk/shop/dogs/c...ood/boos/60631
(Not sure which food brands you can obtain easily in the USA, as I'm from Germany, sorry)
And yes- you may have very unhappy cat during the elimination diet ....
At any rate you are probably better of feeding your cat raw (BARF), as this will be the only way for you to have 100% control over what he's eating.
You should also keep a diary to help you figure out what could be causing the allergy- for example an allergy against pollen will be dedendent of the season. Also keep a close eye on possible fleas, as flee salvia is also a common cause of allergies.
Hope this has helped,