Ok so…where to begin…the topic of getting velocity increased with those mechanics is a moot point.
First, ask yourself if the throw is a push or pull. Then watch the clip over and over until you can see the points in deliver where there is a bad use of energy…lower half, front side…head…whatever it may be that throws off all smoothness and efficiency with his deliver home.
Looks very forced…probably a lot of ‘extension’ or throw the ball out front verbiage has been used. Think more of being in the legs…not tense and squatted…just weight stays over the back half so that when he lands he can land soft and be firm on the lead leg…not soft and then try and push the ball out in front of it. You want a good clean pull against the firm lead leg. Start by keeping his weight on the inner half of his back foot…up the calf on the inner part of the ankle…once he can become comfortable there and keep his head over his tall back knee then he can separate his hands and land soft on the lead foot and pull against. All of this is tied together with an escalating rhythm…which turns into being aggressive through the pull not to it.