It may be that he's actually over striding. He seems to be collapsing the back knee to get the front foot out a bit further. The stride should only be as long as his flexibility allows without a collapse of the back knee. In my opinion, too many people focus on stride length without thinking about how that extra length is accomplished. The stride should just happen and should not be forced. Instead focus on back leg drive and lower half flexibility instead of stride length and lower half strength.
He also seems to land severely on his left heel. While landing on the heel isn't a bad thing, he's really toes-up to the sky when that heel touches. He seems to be able to get up over that front knee pretty well, but he may be able to release a bit further out in front and really be able to incorporate some improved trunk flexion into his finish if he doesn't reach out with the stride and allows his back leg to dictate his stride length.
I would work on two things with him:
1) improve back leg drive by eliminating the back knee collapse
(he may ultimately be able to still land in the same spot via back leg push rather than front leg reach)
2) see how much he can comfortably reduce the severity of the heel-first landing. Try to keep the left foot more toward parallel with the ground and lead with the outside of the left ankle for as long as possible to delay hip rotation for as long as possible.
Is that his throwing hand dangling to his side during the leg lift?
I'm convinced that hand separation speed is directly related to arm speed. Delaying the hand break forces the arm to move quickly through the remainder of the delivery. Keep those hands together through the top of the lift and the arm speed will increase to catch up with the body.
I see a lot of room for increased mph. He's got the frame to add some gas.