Could this be ringworm?

This is in regards to my stray Limpy. When he returned on Sunday morning, I noticed that right across his back, at the base of his tail, his fur looked like it was indented. I couldn't see it very well as it was night time, 2:00 in the morning. When I saw him tuesday morning before I went to work, he had more fur missing. But I don't see any redness or sores. Just his fur is missing. It's about the width of a quarter now.
Does anyone know what this could be?

Is he an unneutered male? It could be what they call Stud Tail. Which are greasy deposits of hormones I believe. But I am not totally sure.

Denyce