I used to have a shih-tzu with terrible allergies. We would bath him in special shampoo that we picked up from the vet. We tried a few different types, the aloe and oatmeal one and the antibacterial one. We bathed him once a week in it at least. He was a small dog so it was easy whereas a larger dog might be harder to bathe often...
I'm surprised that vet gave him the Frontline, and then took him off it What made him think the Frontline was the culprit? We quickly learned that my shih-tzu Ripley was allergic to fleas and therefore he had to be on Frontline since he would get an allergic reaction as soon as they bit him. Could fleas be the culprit and the vet is going after it backwards???

Also, maybe Sandra knows more about staff infections? I think those are pretty serious and need treatment but I dont' know much about them...
Either way, something needs to be done because your furbaby is suffering. As are you on his behalf. Maybe a different vet?

Good luck and keep us updated.