His delivery is a bit segmented and choppy because he's learning to deliver the baseball. It's important that he gets the timing sequences right. His upper half is way early and there is a long stop at the cocked position with his lift leg way off the ground. His upper half is ready to go before his stride foot lands.
At the top of his lift, his hands are already apart. This is the beginning of his sequencing getting out of whack. His lift is also vertical, leading to a back leg collapse to get his body moving forward again. If he gets his hips up and forward, it will extend his stride. Teaching to step out doesn't allow for proper weight shift. It tends to keep the weight back too long and ends up exaggerating the back leg collapse.
His glove side elbow is sagging a bit--get that elbow up so he can sight down the upper arm or reach out toward the target with the glove while his arm is going backward (equal / opposite).
He's also getting to full cocked early. His arm should extend rearward then he should get the forearm up as he's just about to touch down with the plant foot, but the big problem is that is stride foot is still way in the air while his arm is already up. It seems the voice on the camera is stressing lower body importance, but allowing the upper body to be early and waiting for the lower half to catch up will allow none of the lower body energy to be incorporated into the pitch.
I'd work on the rocker drill with his throwing arm extended backward and his glove arm extended forward so he can get the feel of his arm parallel to the ground then getting the forearm up into foot strike. Foot strike and arm cock should be about simultaneous and timed with the very beginning of his hip and shoulder rotation.
Additionally, I would not be talking about corners with a 9 year old. 9 year olds are lucky to throw 50% of their pitches within a foot to each side of the target line. A nine year old should focused on throwing hard. The harder he throws, the better his sequencing will be because his body will work out a lot of the timing on its own.
Targeting corners equals slower arm speed. He's then aiming people tend to slow things down when they try to put a pitch anywhere but down the middle.