My Brooklyn had MAJOR food allergies and I contacted a pet nutritionist and she gave me great advice on what to feed brooklyn.
Here is her article about allergies in case your interested in reading it:
http://www.askariel.com/Articles.asp?ID=256
Sorry to hear your baby Oliver isn’t feeling well. My cat Brooklyn had a really upset stomach and I went to the veterinarian and he changed her diet. I tried several diets and while they helped a little, what really worked was using a probiotic called Power Probiotic...