I tell all the pitchers I work with that when I have them make an adjustment that it will push them out of their comfort zone - it’s not what they’re used to. And even the smallest adjustment can feel huge. While they’re out of their comfort zone, their velocity and/or their control will likely drop off. (This is why you don’t make certain changes in-season.) But they must stick with it until they get enough reps in to make it feel comfortable. Then the velocity and control will return. If you’re not willing to do this, you will have a tough time improving. Nothing improves overnight - everything takes time. I know that can be difficult for young kids to buy into.
I just finished working with a 10yo you had incurred a mild growth plate injury. This doesn’t even pitch - he’s a position player. This kid had a glove issue that resulted in early shoulder rotation and hips and shoulders rotating together. This was easily seen on high-speed video. I’m not trying to scare you and I certain can’t predict injuries. But this is an example of what kind of thing can happen. So I would take seriously the sequencing issue I mentioned previously.
Now, I took another look at the video you posted above and have some more comments for you. First, your son shifts his posture at the beginning of his delivery. The step to the side and back starts the movment toward 1B nd 3B. Then, as he goes into knee lift, he leans back toward 1B. Finally, as his knee drops, he leans forward toward 3B. All of this movement in directions other than towards home plate will be a source of control issues. It will make it more difficult to have a repeatable delivery.
Next, your son’s lower half is not contributing much. He is late in getting his hips moving forward. It’s tough to tell from the camera angle but you can look at his shadow on the ground and see this.
Finally, like I mentioned previously, he has the sequencing issue of hips rotating with the shoulders. This keeps him from using his body to through. The consequence is that he’ll try to compensate using his arm.
Note that I can’t really see what his glove does but it kind of looks like he turns the glove over before foot plant. If that’s correct, then there’s some room for improvement there. Ideally, he should be “equal and opposite” as close to foot plant as possible. That can be achieved by holding the flove out front longer or by getting the hips moving to get into foot plant a bit sooner.
My suggestion is to start with what comes/happens first in the delivery. That would be posture and balance. I would work on eliminating all unnecessary head movement. Have your son start in a more athletic position by putting a slight bend in the knees and waist. Tell him to think “batting stance”. Then have him try to keep his head from moving in any direction than torwards home plate - keep his head upright and online with the target as long as possible. You can use the NPA’s knee drill and rocker drill to isolate the upper half to get him started. Then add in lower half movement.
Once your son has the posture stabilized, then work on getting the hips moving forward a tiny bit sooner. Once he’s got that figured out, work on equal and opposite.