Thank you for explaining!
I had never seen red skin like that before (or a battle trimmed one), so it may very well be from the treatment you describe and not parasites... geeze, what kind of parasite could withstand that. :hmm
Do you know if it makes the feathers not grow back in the next...
He might be molting but by how red the meat on his leg is I would venture to guess parasites like lice or mites.
Suggested treatment is a permethrin based spray sold in the equine department at most places under many names and in ready to use or concentrated forms. The one I use...