As far as I'm aware, Gentamyacin is a very mild antibiotic. Definitely not the kind of thing to use on an abscess. I'll second the injectible penicillans that were mentioned. They're very effective and far better than any oral antibiotic, although you can use an oral drugin combinationwith the injected drug.
I don't have any recommendations for treating the pain. My vet doesn't want people to give pain meds to their pets before a vet visit even if they are leftovers of a previous prescription- correct drug and amount. Sometimes it interferes with their ability to diagnose a problem. Is he eating ok though? If the abscess is connected to a tooth root then he may have trouble chewing. Offer some softer foods like leafy veggies, Critical Care or softened pellets, or canned pumpkin.
Best of luck, and keep us updated!