can you describe it? is it movable? how large is it? is it red or swollen? did he fall? has he been sick recently? is there a rash? where exactly is it?
is it in his scalp? behind his ear? under his chin?
I am trying to figure out if it sounds like a lymph node from recent infection? so I need you to describe the location as best as possible? Is it noticable by just looking at him or do you have to touch it to notice it?
From the location, it sounds like he bumped his head into something and not a lymph node from a prior infection. Since it is not red, or painful there is nothing to worry about. They usually take a few weeks to resolve. He is certainly still at the age that there are frequent falls and accidents and very few of them result in any serious problems.
Certainly if any concerning symptoms develop he should be seen by his pediatrician, but it certainly sounds like he will be just fine and this is nothing to worry about.
please reply with any further questions.
if I have answered all of your questions, have a great night.