Unfortunately none of those pics are clear, so it's hard to tell what it looks like. That is not a common spot for ringworm though, ringworm is more common around the face or privates where it's a moist area that promotes the fugal growth. Does he normally get his tail wet, like sits in his pee or drippy water bottle? More common for a bald spot there is either rubbing the fur off sticking his tail through the bars, or an injury and he is biting it because it hurts. I would keep an eye on it and if the bald spot gets bigger or looks worse a vet visit would be a good idea.