Could be a few things. May be ringworm. Could be a bite of some sort. Did you look closely for any bite or sting mark? Any inflammation, bleeding, swelling? Other then washing the area with just warm water and maybe a mild soap, rinsing and drying well, I would not put anything on it and wait for your Vet to check it.
Could he have got chemicals on his head? Something used to treat fence posts?....etc? Does he like to roll on the ground? Do you put anything on the ground? Or are there any cans with chemicals/motor oil....anything else in them lying around anywhere? Does he like to lie under vehicles?
So many questions I know. But this sounds really strange. I think taking him to the vet is a good idea.