Thanks for posting,
Given the description of the bump, it seems to be an infection which could be due to staph or strep or any other common bacteria that can occur in the mouth. The bump could be a developing abscess
or subcutaneous infection which can be treated with a course of antibiotics simply.
I do not think it is something you should worry much about as within a couple of days it might clear off and your son will be fine.
If this persist, after your son has completed the doctor's prescription, do take your son back to the doctor or consult a pediatrician
for proper review and treatment.
Hope this helps