I'll throw my two cents in as well; it's possible there's something in the home environment hat is acting as an irritant to kitty. You should do a careful review of all household products used and anything unusual in terms of carpeting/furniture/deodorizers etc...
Be diligent! This type of problem can develop into long term scarring that causes permenant issues.
Has your vet done a culture to see just what type of bacteria you're dealing with here? If not, then at this point it might be a good idea. It could be that the strain that is afflicting your cat could be resistant to antibiotics, and they're knocking it back enough to make the cat feel better, but aren't knocking it back enough that his immune system is able to beat it.
Ghilly