What kind of food are you feeding? A good hypoallergenic food won't contain corn -- but alot of dogs are also allergic to wheat and rice. I'd suggest a primary or fully homecooked diet. Try adding some oil into his diet (not canola oil!!).

Why do you need to bathe him so often? Bathing makes the skin drier because you are getting rid of all the oils that keep his skin moisturized. Dry = itchiness. But if you really feel that bathing him helps, try bathing him in oatmeal. Oatmeal is very good for itchy or irratited skin.

Good luck! Try researching all of the vitamins that are good for the skin, because I can't remember them lol.