It could also possibly be lice. (gross, I know, but...) One of mine was rubbing to the point that he had huge bald spots on both sides and all down his chest and neck. I never saw anything on him, but had the vet out and he found lice on him. They weren't too hard to get rid of, but how to treat them depends on what kind, so it would be something to talk over with your vet. Hopefully it is an easy fix and he is fluffy again in no time. :)
At some time in their lives, every dog deserves a little boy, and every horse deserves a little girl to be the center of their world.