Best guess from a picture on the net is a infection of some sort. He could have scratched the inside of his mouth with a bit of bedding, or the rodent and it's just now getting swollen with fluid or pus.

But it COULD be several other things, like a cancerous tumor, embedding foriegn body, etc.

Always best to have a good reptile vet look at it. If you need antibiotics or antifungals, etc, you'll need to go to the vet anyway.

Good luck with him. And I'd quarentine him away from the other snakes, just in case.