This is totally a hunch, but... check him for mites?
The reason that I say that is that the only time I've remotely seen anything like that was in my cats, and it was self-inflicted--scratched all the fur off. Right behind the ear is one of the few places that chicken can reach to scratch at with their feet, and I've seen mine accidentally claw themselves before. If he's itchy, he may have scratched his feathers out.
First, see if you can see any crawly things around the base of his feathers.
If he doesn't have parasites, then assume that it's just an irritation that's being aggravated by the other chickens noticing the red, bare skin. You can treat it with Blu-kote. It will help heal it while coloring the bald spot a purple color, which is neutral to chickens, so they won't be attracted to it, pecking at it, keeping it raw.