I had a pony at one point who rubbed himself raw on his face, back and everywhere else. We discovered he had sweet itch. Put Listerine and baby oil on him daily, along with fly spray, and super fine mesh fly sheet. Turns out he was allergic to gnats. The real cure on this for us was the Listerine and baby oil. So maybe try this instead of the Listerine and apple cider vinegar. We also kept him in a hot wire field with no trees to scratch on. If it doesn’t improve, the vet may be the best next step. Hope your horse gets better.