Feathers missing on a Roo's back?

[sweetpeace]

Our chicks are about 8 weeks old and I'm not sure if this one is a roo, but s/he has a more pronounced comb than the other 4, so I assumed. S/he is also the first one to the door and will peck at anyone trying to get in.
But just a few days ago we moved them from the house to their outdoor coop and today when I went to hold him/her, I noticed that his back is all bare and rough.
No mites that I noticed. The cage was thoroughly cleaned before using.

What else could it be and what could I use on it?
(I included the pic with the comb showing comb so you could tell me what you thinkn or cockerel)
Thanks so much!

 

[Eggcessive]

He may be getting overgroomed by the girls on the roost at night. A cream called Nustock (sulfur-based with pine oil) from the feedstore may be good to apply--it tastes bad and is healing. Look under his vent area for mites or lice and their eggs at the base of feathers.