Thank you for your query.
For how long has the bump been present and is it increasing in size? By the description provided by you it appears to be urticaria
. However urticaria usually comes in crops. Apply some calamine
lotion and observe for a day or two. In case it increases then it shall be advisable to consult your doctor.
I hope I have answered your query to your satisfaction.
Wishing your son all the best.