What's wrong with my dog?

He has small scabs all over his back that come off if you pick them. He constantly licks his lower abdomen that becomes raw, then turns hard, black and crusty when it begins to heal. The inside flaps of his ears have also started to become hard crusty and black. He is constantly itching. He's been tested for fleas, mange, parasites, and we even tried putting him on presription hypo allergenic food diet. He's been on antibiotics multiple times, which cleared up the scabs, but then once off of it for awhile, the scabs reappear. Something is wrong with him....please help!