I might suggest the following adjustments:
(1) Minimize the size of the initial rocker step to the side. That creates too much movement in the wrong direction.
(2) Increase the tempo of his stride. Striding faster will stretch out the length of his stride a bit and let him put a little more zip on the ball.
(3) Have him try to keep his head upright through release.
I'd recommend having him work on only one of these at a time.