I hope the Benadryl helps him.
If he is not on flea prevention, Frontline is a good one, then he should be.
If he has a food allergy it may take a while for the new food to help.
You could see if a bath with a chlorhexiderm shampoo will help. Sometimes secondary infections from initial trauma are more itchy than the thing that started the problem.
You can use a lidocaine spray on the really itchy spots which should give short term relief.
If in the vets you have seen neither was a veterinary dermatologist you might want to see one for the best expert in person help
http://www.acvd.org/site/view/55607_FindADermatologistAlphabetical.pml
You can read up on allergic dermatitis here
http://www.lbah.com/allergy.htm
and yeast infections of the skin here
http://www.marvistavet.com/html/body_yeast_infection_of_the_skin.html
Hope this helps you!
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