What Causes A Hard Knot On The Forehead?

My son is a lil wild, he has hit him self on the head numerous times. From falling and hitting it on a lil scooter. He tends to always hit himself in the same spot and the knot is the size of a silver dollar every time. Should I take hime to get X-rays since it is the same spot always. Or is it just a random act that he hits the same spot. And he does not change his attitude or demeanor after his accidents. He's still spunky immediately after?