What Causes A Cyst Like Bulge Of 3cm On Mid Of Forehead?

My 15 year old son has a cyst like bulge about 3cm in diameter just to the left of the midline of his forehead - it goes up to his hair line. It fluctuates in size a bit and pulsates. I am concerned that it is in some way connected with an indented line that can be felt in his forehead, going down to his nose from the hairline, as the bulge is just to the side of this. It is soft to touch and does not seem to have a centre. Are you able to advise on this? Thank you