You know, maybe he tried to defend the flock, and some of his feathers were yanked, and now his skin is irritated......You just never know. Once, over winter, when my small flock was cooped up quite a bit, everybody (even the rooster) was getting a bald spot just above the tail. I looked, like you have, and no parasites. So I just saw with them, watching them, and I saw it. The SAME hen would sneak up behind each chicken, and WACK! She would pull out a few feathers in the same spot on their backs. And her back was perfect. No bald spot.
I don't know why she was so mean, but I sold her. Everybody grew their feathers back.