Well, do keep an eye on it. In case you don't know, snakes form mostly solid pus from infections that doesn't usually drain away on it's own, but instead tends to form a firm lump (abscess) for which the only solution is surgical removal. If that's what this is, you wouldn't necessarily see anything inside his mouth- it would be sandwiched in the tissues between his outer skin & the inside of his mouth. Snakes that are prone to pushing on their enclosures with their noses tend to end up with an abscess on their nose, for example. But I do hope this is just a false alarm.