The issue may be coming from how his weight is distributed across his feet. Make sure he's keeping his weight back toward the back of the feet in the first pull and all the way through the second pull. Be careful he doesn't exaggerate the this part of the movement and overcompensate by rolling to his toes too soon. He needs to think about extending while his weight is still back and open up as much as he can at the top.
Let us know how things go or post some more videos if you want.