I see don't Fowl Pox, can you point that out in the photos? Do you mean he has sores or lesions inside the beak? Photos of that? May I ask why he was separated and in a super dark pen? To me he does looks like he may be molting too - feather loss, no tail feathers unless they are obstructed somehow in the photo. I agree, I would treat him for the lice asap. Get some extra protein and vitamins into him.