Are you using a flea control product?  The distribution (dorsal, and particularly the base of the tail) sounds like flea allergy dermatitis (FAD) until proven otherwise.  Some dogs are intensely allergic to flea saliva.  You may never see the flea because the dogs nibble it off, but it may only take one bite for them to erupt into a scabby itchy mess.  Even if you don't see fleas, get some frontline or advantix on him and reassess.