Sidearm Overstriding?

When throwing from the side or just a bit lower, is the stride shortened? My son (before dropping down) was striding close to 80% of his height. (He has been throwing from the side for a while with increased accuracy and velocity without pain)

The problem I notice is he is not getting over the front leg and the ball is released around the time his front foot lands causing the ball to arc almost as if he is pushing it. If he shortens the stride, the ball is stays low and runs nicely.

He throws a 2 seam, Split-finger (Change) and oddly enough, WHEN he shortens the stride and follows through, a little Slurve. If he does not shorten the stride and throws the Slurve it spins like a bullet directly at or behind the batter with ZERO break.