Bald spot on cat's back.

My friend's 1 1/2 yr. old cat has been losing the hair from it's tail for about 10 days now. He was an indoor/outdoor cat who roamed a lot. She said she mainly fed him dry food, but when he'd come to my house I'd give him wet food. I had him neutered for her on 4/13. He stayed at my house before and after surgery for about 3 days in all, and my 2 boy cats sort of terrorized him. Then he went home with her. Also, he had impregnated their female cat who had kittens at the beginning of May. She has the mom and the kittens separated from him. She continued to let him outside after he healed. Then she went on vacation on about 5/12, and I've been taking care of the cats at her house. A few days before she left, I noticed about a 2" bald spot on his tail which has increased over the week. It's about 4 - 5 inches long now starting at the top of his back. I'm feeding him wet food and dry food. He has a good appetite, and his stool is normal. But I'm keeping him inside and not letting him get near the mom and kittens. He doesn't seem to be distressed at all, except that he wants to go outside. Other than that he's acting totally normal. I also gave him an Advantage flea treatment on 5/16, and I've been applying hydrocortisone cream and aloe vera juice (just a little) thinking that fleas might be causing the problem. The bald spot was already there when I started these. The mom and the kittens have some fleas, so I figured he probably did too since he was going outside. His skin on the area where the hair has fallen out isn't crusty or discolored or anything. It's perfectly smooth.

Any suggestions? I'm worried about him.