Thanks for the picture; that does help!
This could be a skin infection, an allergic reaction to a bug bite, or it could be a small patch of "ringworm", which is not a worm but a fungal skin infection. Has he been around any young cats or kittens recently?
If this were my own cat, or a patient of mine, I would shine a Wood's light (ultraviolet) on this, and see if there is any green glow under the light. If there is, this is ringworm, and it should get better with a topical antifungal ointment like 1% miconazole.
If it does not test postive as ringworm, there is still a chance it could still be ringworm. I might take a few hairs to culture for ringworm, and start treating with topical steriods, antibiotic and anti fungal ointments.
Or you could just monitor it, and see if it goes away on its own, which it might, and if it does not, have it tested for ringworm.
Is it itchy to him?