Recent Balding above tail!!

Please Help Us!

About a month and a half ago I noticed a white spot on the top of my outdoor cat's behind, I let it go because I thought the hair was just pushed around. About a week later I noticed the spot got a little larger. For about another week I watched my cat and the spot. I didn't notice him overly gnawing scratching himself, but the spot got a little bigger and a few other spots on the side of his hip started appearing. And when I try to get a close look at it and touch it, he either moves, growls or runs away. I took him in to his veterinarian and she told me he was getting bitten by fleas since she saw a FEW on him. She told me to put some Advantage on him and it would get better. Although it has only been a week since I put the Advantage on him, I am still concerned because not only is it not getting better, but it is getting progressively worse. Plus, I have not noticed any fleas of him or my other cat.
Additional Info:
His mood isn't changing, but he has pooped a few times on the carpet in various corners of the house, and once right in front of my boyfriend.

What do you think it could be, and what should I do?

Here are some pics of the spots:

Spot above his tail on his bottom


Spots of his side and leg


Overall of the spots of his side

the pictures didn't show up for me, but it might just be my mini dell.

My cat had allergic reactions to flea bites and lost hair above her tail and around her neck/shoulder area. We gave her advantage and it took two doses to get rid of them. I know mites can also cause hairloss too.

I know both hyperthyroid and hypothyroid cause hair loss but I am not sure what the hair loss patterns look like.

I can't see the pictures either. My cat Princess has severe flea allergies and when she has problems with it she loses hair in various places. The most common place is above her tail. It takes Princess over a month to start regrowing her hair. She also gets litttle scabs in the area. I have to apply advantage every 3 weeks to help prevent fleas. There is also a medication called capstar that your vet can give you that will kill all the fleas on your cat in 24 hours.

I think you said it's only been a week since you applied the advantage to your cat, If that's the case I would give it a little more time. If your kiity is having new symptom I would call your vet because it could be something more than fleas