Isolated areas of Dandruff

Question:
 Hi,
We adopted a 9 year old chocolate in February. Jake is a wonderful dog and we feel truly blessed to have him. Poor guy is allergic to just about everything. He is on HA (soy food) and seems to do quite well. A couple months ago I noticed a place on his side about 2 x 3 inches in size. It was a very dry spot and if you scratched it, big flakes came off. He is gradually getting more of these spots - a couple on his back, one on his neck and one under his arm. He pays them no attention, but if I use his brush to sort of scrub the chunky dandruff off, he reacts like they are very sensitive. His hair comes out in little clumps attached to this chunky dandruff and the skin looks a little bit red and irritated where the dandruff came from. Eventually he is losing all the hair in the patches where the dandruff is. We give him fish oil twice a day and he does not have any dry skin or dandruff anywhere except in these isolated spots. We have been dog sitting my mother in laws dog for almost 3 months as she has been ill. I did catch Jake sneaking some of the other dog food once so its possible he gets a few nibbles occassionaly when we aren't looking. Maybe its an allergic reaction to the food. Any ideas what might cause this problem?
Thanks :) Jamie

Answer:
 I don't have much to offer, but I have heard that lots of dogs are allergic to soy. Maybe that's what happening with your dog?
Have you considered environmental allergies as well? Does benedryl help?