Have him try this:
First, grip the ball with thumb and middle finger cutting the ball in half.
Then, if he has the flexibility to tuck the finger along the side of the ball to make a circle without allowing the thumb to creep up the side of the ball towards the index finger, then do so, Otherwise, just tuck it as far to the side as he can. He might make a "C" instead of a circle. Most young kids don't have enough flexibility nor big enough hands to make the circle.
Finally, pronate the hand/wrist/forearm enough to put the middle and ring fingers on top of the ball and throw it like a fastball.
That usually will take enough off the pitch. You just need to make sure that he throws through the ball and doesn't "swipe" across in front of his face. The pronation tends to make kids swipe. If he can get this down, then he'll beon his way to geting some movement on his change-up since the pronation imparts force around the ball and puts spin on the ball.