I trust Beekissed nu stock advice....find some and put on your roosters butt. In the meantime, I hope someone will chime in on what's causing this......definitely check for mites or lice. Use wood ash to dust him with...........
Have you by any chance treated him recently for lice / mites ? Used any chemicals in or near the coop? He may well be having an allergic reaction to something he is getting access to.- he could be dust bathing in something i.e. wood ashes - if he does have access to wood ashes they must be cold as he could be getting burns from hot ashes.
These are just suggestions for you to consider - he should have some antibiotic cream applied to the area as he risks infection.