Having this same exact problem with my 14 y/o brother.
First thing to look at is head positioning. Keep the eyes still and on target through the entirety of the delivery; take the nose straight to the catcher.
In the lower half, try hooking the rubber with the back foot (pigeon toe). Get the back knee inside of the ankle early, like during the descent out of the leg lift, and work down into the landing. Continue working on holding the front hip closed into landing. Work the rotation of the lower half from back to front; it will sync things up a lot better. Look up “rotational lateral bounds” on google to practice this movement.
Let the glove side work with the front leg. Try working north and south with the glove vs. east and west (pulling). Use the elbow to initiate up and swiveling back into the rib. Look up “pivot pickoff” throws on YouTube, to isolate the upper half and learn how to efficiently connect the glove arm with the throw.
The biggest thing I’ve learned is the length of the stride doesn’t mean anything if you can’t stabilize it. It looks to me like his stride length is near 100% of height. Experiment how far he can stride and hold his landing, work from 50% up, till he can no longer stabilize. I bet 85-90%, and he’ll be able to stabilize and control his finish. A drill I’m putting into practice I call “stand ups”. Knell in a split stance, close off your shoulders, initiate your glove, throw the ball and stand up into your finish. This will help teach him how to control his entire body to land and decelerate properly. One miscue like an overaggressive glove or head movement will make him fall off like he does off the mound.