What sort of anti-biotics has your vet given you? I have heard of a oral antibiotic, which you rub on the infected areas inside the mouth (gums etc), but for the life of me can't find out where I read about it. Maybe your vet would know?
Also I remember reading about diet, this can more often than not contribute to the disease. Has your vet said anything about this too?
What do you feed him? There are certain grains that have a high sugar content etc, this can add to the problem, might be worthwhile speaking to your local feed merchant, they might be able to advise you of certain feeds that may help.
Sorry to ask so many questions, but it can't be nice for either of you. Xx