Just a thought. You may want to double check his grip. Check it on video or watch closely without telling him what you are looking for.
Young guys tend to tuck the ball into the palm, which deadens the ball, reduces the spin somewhat, and are throwing, in essence a change-up.
If the ball is not buried in the palm, check to see that his ring finger is not tight against the side of the ball, which will also affect spin and velocity. If his hands are small, have him try getting three fingers across the seams and getting the ball out of the palm some. He will get more spin and velocity may go up a notch since he is not clutching the ball with the ring finger. Keep an eye on where he positions his thumb also, likely its alongside the ball.
Good luck.