JD mentioned earlier that having the 2 fingers together, or maybe it was SouthPaw-Dad, is best when released. It will come off the hand more smoothly and most importantly, 3 fingers will create more friction during release. 2 fingers, with the longer middle finger releasing last will reduce friction and will maximize spin and velocity. There's no way in my mind that his velo won't change with the 2 finger grip. If you're not sure, gun him.
Then again as JD said, what level is this kid going for, will he be a successful high school pitcher, does he have the stuff to start on his high school team? With the change, will he have the chance to play college ball? Things to consider before tinkering, but the kid should know and understand that he's doing it just plain wrong.