Ugh, allergies are SO frustrating...believe me, I know. I have 2 that deal with them. First, food allergies are very, very rare - so I highly doubt there is one. That said, it can be supportive to have an allergy dog on a grain free or low-grain diet as it's considered less inflammatory to the system in general.
To me, it sounds like he has enviro allergies (ie, pollen, dust or whatnot). You first need to figure out if these allergies are year round, or seasonal. If year round, you'd want to treat w/ OTC antihistamines (rarely ever work), Temaril-P, Atopica (very good), Apoquel (also good), or do allergy testing and desensitization. If seasonal, you'd treat w/ OTC's, Temaril-P, steroids, or even Apoquel.
Benadryl and other OTC's are VERY rarely ever seen to work, but they sometimes do. But w/ Benadryl, you'd have to give it 4x a day as it's half-life is super short and it just wears off. The OTC's most Vet Dermatologists recommend are Zyrtec and Claritin bc they last for a full 24 hours. Also, you always want to try any OTC med for at least 10 days before assessing it's effectiveness.
To me, it sounds like your kiddo has recurrent pyoderma (ie, staph intermedius overgrowth on the skin) - which is usually treated with Simplicef for a full 30 days to get rid of it. Is this the med you've been using? If not, that's what I'd be asking for.
Also...are there ANY other weird symptoms? Some dogs who get recurrent skin infections actually have hypothyroid or Cushings...so keep that in the back of your mind as you go forward.
My very best advice to you is to get to a Vet Derm ASAP. You can either spend hundreds and hundreds at your generalist vet (this is what I did...sigh) OR, go right away to a Vet Derm and get squared away from the get go. Vet Derm appt's will be more upfront...but will cost you WAY less in the long run. Otherwise, please def ask your vet about getting on Atopica or Apoquel if these are year round allergies.
Lastly, get this shampoo, it's what Vet Derm's recommend but charge 4x this price at the vet office:
Amazon.com : Douxo Chlorhexidine PS + Climbazole...
And read this thread for more info on how to approach allergies in dogs:
http://www.yorkietalk.com/forums/die...tart-here.html