Watching it in normal speed I'm wondering why he lifts his leg up, down, and then moves out. Then you watch it slowed down and you can see a few things.
He moves his front leg up and back, which means he's going to have to start hip rotation a bit sooner to land normally (which tends to pull the back foot off the ground before ball release). This takes away from velocity but also control (as throwing off one leg isn't as stable as a two leg base, which can also set someone up for injury)
This compounded with him not getting good leg drive off the mound (it looks like he's moving slow, not keeping his hips closed long enough [because of the early rotation], reaching out with his leg), all of which hurt his velocity.
So I'm comparing him against a slow motion video of Justin Verlander, which looks a little something like this:
And
Compared to:
So you can see Justin is doing a better job at not opening up as soon, getting good leg drive, his nose isn't out in front of the middle of his body (which happens when you lead with your foot and rush and things like that instead of driving with the front hip, getting good back leg drive, however you like to phrase it). So I would definitely work on those things.
I'd say eliminate the up, down, and go thing, but if that's too extreme or he's too uncomfortable doing that before the season starts then okay, but definitely work on staying closed a bit longer, get rid of that early rotation because of him turning his front leg a bit towards second, get him driving his front hip aggressively towards home plate. Show him those pictures so he can get a mental picture. Go over the video with him so he can see what he's doing. All of these devices can be important learning tools.