There are a few things that can cause this:

Standing up (straightening his right leg) in his backswing and dropping back down on the downswing. (This sounds like it is the cause, but it could be simpler and be the following two)

Standing too close to the ball.

He is coming too steep on the ball.

If the ball is only going 10 yards, he is hitting behind the ball. Get him out to the range and watch to ensure he is hitting the ball first, then the ground.