Advice for itchy/scratchy/maybe food allergic cat?

Hi all, I was wondering if anyone had advice about my itchy cat, Peanut.  About three weeks ago, he started developing a dime-sized itchy patch on his shoulder that was red, crusty, and balding, along with two or three other small scabs on his body.  I took him to the vet who prescribed a week of antibiotics (clavamox) and prednesolone for the itching.  We adopted him in November and recently moved to a new house across town in December so the vet though maybe it was just a manifestation of the stress of the move.  By the time he was done with his medications at the end of one week, the smaller scabby areas had healed and the bald patch was growing back hair (yay!).  But as you can probably guess, a few days later the itchies returned, even worse this time.  He has a pencil-eraser sized patch on his "eyebrow" area that is red and balding and the skin on his shoulders and in between his shoulder blades is red and oozing (although no hair is falling out there).  I called the vet back and she said that maybe it is a food allergy and to change him to a different protein source, so I picked up the Nature's Balance LID Duck and Green Pea, which she okay'd.  (He ate this  Blue Buffalo food for maybe two weeks when we first brought him home from the shelter before I switched all of the cats over to this  Nutro Max Cat for probably three weeks before the itchiness began.  They were also splitting a can of the matching Blue Buffalo wet food.)  Peanut is also on another round of antibiotics, this time for 14 days instead of 7.  No prednesolone this time though.  

Here is my ultimate question - if you have had experience with a food allergic cat, how long did it take for him/her to respond to a new diet?  Peanut has not seemed to improve at all after 10 days on his new diet and day 5 of the second round of antibiotics.  The vet said that you have to give the new food 8 to 12 weeks before determining if that is the cause, but he is still scratching at his wounds and making new ones.  He doesn't seem to be incessantly scratching or seem to be in too much discomfort...I would say maybe a 5 on a 1 to 10 scale of itchiness.  I hardly see him scratching but I know he has to be doing it because his skin is still inflamed.  

I should also mention that when he came to us from the shelter, he had a tapeworm that we treated (although I never found any segments or remains in his poop like the vet said I might) and he was also just given a general de-wormer for everything else in case we missed something, even though his fecal came back negative. 

Sorry for being long-winded...any help or advice if you've been in a similar situation would be great!

I'm sorry to hear he's having skin issues. I don't have any personal experience with food allergies in cats. I interviewed a pet nutritionist once and she explained that not all foods marked as hypoallergenic truly are. Some cats are sensitive enough to be affected by traces of the allergen. These tiny amounts can simply be there because the same production line was used for a different batch earlier that day. 

I would consider consulting with a pet nutritionist. That would be a veterinarian who specialized in nutrition and knows more about how to construct a food elimination diet. You can find a list of certified nutritionists in the US here - 

http://www.acvn.org/

Some of them may be available for phone/email consultations. 

I don't have any experience with a food allergic cat, but my cousin has a dog with a wheat/gluten allergy. I'm suspecting that's what your cat has. Looking at the ingredients in the Nutro food - Corn Gluten, and Wheat Flour are the 2nd and 3rd ingredients. I'd eliminate grains first, before I'd look to proteins. It's entirely possible that will fix your problem. 

The new food you're feeding him doesn't contain grain or gluten, which is great, but the first ingredient is peas which is just a carby filler. I'd try to find another grain-free, Limited Ingredient food with meat as the first ingredient eventually. They're out there, in both wet and dry foods. 

The dog is on a raw diet now and that helped, but the biggest thing was eliminating all traces of gluten that he could get ahold of. That means making sure your other cats (if you have them) also are eating a grain free/gluten free diet, and any treats you give him also don't contain grains. With the dog, it did take about a month for his itchy bumps on his head to go away. They did give him Benadryl for a while, which helped him not be so itchy. I'd check with your vet to see if there is a kitty version of this. If he does steal a bagel or other gluten filled treat by accident, he breaks out again, and they have to treat with Benadryl. 

The allergy very well may be a protein, but I'd try eliminating grain and gluten first. 

Your Vet is correct in that is can take many, many weeks in an elimination diet to see if it works

Are the sores open wounds now, or not?  If so, keep them clean using, perhaps, Hibiclens, and maybe even put a solution of Colloidal Silver on them to help them heal. It seems to work quite well.