Did your vet put him on an antibiotic? If its an infection/abscess it will need an antibiotic to cure it.
Over the years I've run into lots of mouth cancer, benign tumors and infections. We had one about a month ago quit eating, became lethargic and her lower right side of her jaw/mouth swelled greatly. Took a fine needle aspirate of the swelling and discovered no abnormal cells but did find some signs of infection. A round an antibiotics & pain medication was prescribed and so far it appears to have cured her.