Hopefully someone else can chime in if they've had a similar experience. It's just difficult to pinpoint the cause of a rash. Several years ago, I tried to treat a skin issue with some ointment someone recommended, and the issue just got worse. A phone call to a vet might net answers or at least help you to eliminate possibilities, and you could avoid the vet visit charge.
In any case, I'm glad he seems energetic, and I hope the rash goes away quickly.