Thank you for your query.
You need not worry for the lymph
node if it is not increasing in size. You should worry only if it is increases in size or is painful.
Just for your knowledge lymph nodes, commonly, enlarge following an infection in the draining area. Your son might have had some injury/abrasion in the back of his head or neck which must have enlarged this lymph node.
I hope I have answered your query to your satisfaction. Please feel free to revert back in case you need any clarifications or would like to share more facts.
Wishing your son all the best.