You may lose him if he doesn't go to the vet's. If there is pus it is infected and he needs antibiotics, and there is a chance that if this is not treated he could get a blood infection (septicemia). Antibiotics are not expensive, but repairing his mouth may be.
check with your local vets.we got a price from one for about $500 to remove an incisor, but the cat specialist vet charged only $120!