What’s this is the mouth?

My son had something similar once, a small lump on his gum, and I was so anxious until our PD had a look and explained it to us. Since we cannot really tell just from a description what it might be, I would bring him to see the PD or a dentist soon, just to be safe and get a proper check done. Even if he seems comfortable now, it is always better to have a professional take a look rather than guess.