Check the bath water and sink or whatever you soak him in for mite evidence. Tiny black specs, mites or mite poops. Also if you feel scabby type area on his belly-bottom area it probably is from mites.
2 of our boys had mites bad before we realized it, they lost feathers around the vent and were red and scabby under feathers. They were not that red however.
Mites can make chickens very uncomfortable and eventually will cause health problems. They also seem to come out of no where and cause problems fast. If you see evidence spray all your chickens. The easiest way to check is to look outside in the sun, they are hard to see. Look around the vent, under side of neck, under wings. The scabby areas is sing you have pretty bad infestation. Spay everyone down in 10 days as eggs will hatch again.
We use frontier spray for dog and puppies, one spray just below the vent and for muffed birds 1 spay in the muff.