[sweetbabyblue]

Hmm..Is your cat white or has a white/light coloured nose? There are many possibilities for lesions, some more likely than others, but a vet would really have to see it to narrow the list down.

In regards to the hair, I think that there is a high possibility that it is an irritated area, possibly itchy.
It is not always possible to find the flea, unless there are high numbers of them. You would be more likely to find flea dirt. It's still worthwhile to vacum the house in case it is fleas and give him a flea bath. Even one flea can be problematic.
Another possibility is demodex, a simple skin scraping carried out by the vet may help to determine whether that is a possibility or not. I am not sure whether demodex is common where you live though compared to Australia.

Certainly another visit to the vet can help, have all the information you've given here written down, because the more information you can provide the vet, the more he/she is equipped to solve this problem. Some pieces of information, not matter how little they may seem to you can actually be very important.

Good luck!