gimages,
Does he swing down on the ball too much? Low sinking pitches can also cause too many ground balls.
Not staying in the box could also contribute to problem. Swingplane of box is tilted downward too much which causes swingplane to create flate disc that tilts wrong way. Disc should tilt in upward fashion in swingplane and in finish. Not staying in swingplane in finish can mean a couple of things> bat is too heavy or recoiling bat and not following through with nice high finish in swing.
If bat is too heavy, the batplane could tilt downward causing all hit balls to become ground balls.
Recoiling and not finishing swing also possibility.
Not keeping bat on swingplane in high finish after contact.
Hard to analyze without clip but bet I'm close even without clip>
Could also be the coach is screaming linedrives/groundballs too much in third base coaching box-LOL
Shep