Skin Problem?

I bought a 5 Y/Old gelding in October. HE had some signs of hair thinning around his shoulder on either side(same place)! one either side just behind his ears and again either side 10 inches back from his ears! After 3 months the hair still had not grown back. (I thought it was chafing and head coller rubs! One looked a bit strange so I put some 3 antibiotic cream on it and another place, and within a week sores were visible where the cream has been! Obviously a reaction!! He does not scratch it! The places are not hot, or red! I live in a remote part of Spain. We have no horse/mule Vets near by just a small animal vet! Blood tests would take 2 months! Please, answers from experiences would be appreciated! Medicine brand names are different in Europe to USA, but please try! Thanks