my dog had something EXACTLY like that but on the underside of his elbow.
i took him to the vet, and they stuck a needle to extract the cells, and got a call a day or two later saying it was BENIGN cancer.
told me that stuff like this USUALLY subsides on its own, but told me since it's on the elbow, and since dogs lay down on their elbow, it might irritate him in the long run.
luckily, it went away in about a week and a half later.
your dog's growth under the mouth would most likely irritate him just the same, since when dogs lay down, their put their heads on the floor, and that growth will be in direct contact with teh floor surface.
anywyas, good call on your part to take him to the vet.
good luck to you and your dog