I am dealing with an issue I've never seen or heard of, and hoping someone has insight....

Our horse that got hurt last year in the bad accident returned to us in June. When he returned, he was on the fat side, but otherwise appeared healthy. I also noticed he had a thin tail. I saw him rubbing his tail a bit, indicative of pinworm, so I gave him the usual treatment. Rubbing ceased, and I've haven't seen any sign of worms since. Now that he's been here a couple of months, he has lost weight, started working, is toning up, and he's looking really good. His color is also darkening back up, which may be indicative of a copper deficiency being corrected, but it may also have just been sun bleaching in FL. The tail, however, is not improving. I have only seen a tail this thin on an Appaloosa horse. That's what made me wonder about copper deficiency, but there is still no sign of regrowth. This is not tail rubbing or broken hairs. It is thin hair, as in you can almost see his skin through the hair at any point on his tail. It is strange. Any ideas what might be causing it or how to correct it?