My Son Is 18 Years Old And Has A Knot

My son is 18 years old and has a knot in the back if his head towards the the base of his neck. Not too far down. It's a pretty good size. He often has bad headaches but now more often. He's had this for awhile now. He saw his doctor for it and said it was probable just his bone and he was growing and should go down. It never has. Any advice please????