Sure, but you can get hurt. I'd love to be wrong, but you can't tell a kid to turn down that opporunity to "get better." And you can always go back and finish school.
But you can't go back and experience the senior season that you missed. That means something to some individuals. Making decisions solely (or primarily) on the basis of the financial implications is a mistake. Life is not about accumulating the largest possible sum in your bank accounts by the time you die. It is about enjoying the ride. He should turn pro if it has been a lifelong dream and he is excited about the new challenge, not just because it is the right financial move.