Under the mouth like maybe the lymph nodes or does your pup have bad teeth or anything? You should give your vet a call, it could be an infection or abscess or strangles or something else that needs a veterinarian. If you don't see any other signs of illness, behavioral changes, trouble eating, swallowing, breathing or anything (anything else) you can probably just wait and call them when they open so they can talk to you about it and ask questions and get answers so you know if you need to go in or you may just want to go in for an exam to be sure. Hope your baby is okay.
A parasitic infection....does he drink out of a toilet?....did any one in your family has had a parasitic infection