I'm not sure how I missed this thread. Sorry for taking so long to reply.

If you're positive that the fleas are gone and the bald spot isn't an allergic reaction to the fleas, then your cat could be allergic to something else. My cat Creamsicle had the same thing except that it spread to her tail and all along her spine and head. She lost so much hair that her tail looked like a rat's tail. She's now on Prednisolone because we haven't been able to determine what she's allergic to and the itching was driving her mad.

It's a good idea to take your cat back to the vet and explain the situation or you might also get a second opinion. Has your cat lost more hair since you posted this thread? Also, seeing that it's an outdoor cat, he could be allergic to all kinds of things.

Good luck and let us know how s/he's doing.