I Just Noticed A Hard (bone-hard) Bump On The Back

I just noticed a hard (bone-hard) bump on the back of my 14 yr old son's head...I don't have one there, my husband does not either. It is located just off-center of the back of his head, approx. 1-2 inches above the hair line. Is this something normal or something we should be concerned about? He states that it's been there for as long as he can remember and it does not seem to be getting any bigger and does not seem to cause any pain/discomfort.