It's rediculous. How can the hair grow back thicker if you keep shaving it off? That's a myth. When you get a haircut, your hair does not grow back thicker. It's the same as before. You are born with the hair thickness, that you were meant to have, and there isn't much you can do to actually change that, though alot of hair care companies would like to tell you otherwise. Tell him that.

I agree with raven53. I don't think there would be a medical evidence for that.