Has he been itching the area at all? If not, then I'd worry about fungus or, possibly, mange. It could be ringworm (a fungus) - and you won't often see a "ring" like in humans. If it's not related to itching, then I'd probably ask for a skin scraping (for scoping) and then grow a fungal culture. Aw Channing....like you need one more thing to worry about.
