It is difficult to assess the nature of this "lump" with the information given. Is it hard like a marble or soft to the touch? Does it move freely with palpation or is it fixed in position?
Has it grown in size since you first noticed it or remained the same?
Possibilities range from a cyst to a lipoma (growth of fat cells just underneath the skin).
Even though he is not experiencing pain, it would be beneficial to make an appt with his pediatrician for a medical evaluation. Once the physician has an opportunity to palpate this lump, he will have a better ability to determine its cause and what treatment (if any) is needed. It is more than likely a benign problem, but it is always reassuring to have a specific diagnosis.
Best wishes ----
Thank you for the respond. I do have an appointment next week.