Sorry, I was thinking of a different thread and thought he was not neutered, sorry my mistake.

If it's what I think, around here we call it 'stud tail'. Jack had that when he showed up here. We had his hair clipped, took him to the groomer and had him neutered and it went away. He now has a beautiful very full tail.

I hope your kitty's gland settles down and doesn't cause anymore problems.