He catapults the ball. But before that....
First -- Good stuff
Takes front hip toward catcher before hand sep. YA!!!
BUT... then... welll.... here is a few thing I can see.
I am not an advocate of the amount of rear knee bend he is doing. You do need to bend the rear leg, but his seems way too deep.
At 18s in the video you will see that his throwing hand is above his shoulder and head before foot plant. Way before foot plant. Planting the front foot is the mechanism for transfer of body energy into hips/core and moving arm from elbow to wrist into a position above the shoulder and for hip rotation to begin. He is way too early.
You will also see his glove high position. We would really like to think of the front arm, from shoulder to elbow as the barrel of a gun with a "sight" mounted on it. Because of this glove high position, he is pulling down. All of our movement either needs to be forward/back in the direction of the catcher/pitcher line or rotational around in a vertical above that line.
Our bodies want to counter balance us. If we swing one arm up, the other wants to come down and visa-versa. We want all of our momentum during pitching to be directed towards the catcher, not up and down.
Probably because of this countering effect his body is doing, his throwing arm goes straight up after initial arm bent position and forward motion in a catapult motion. If you do stop frame by frame starting at the 18s point through release of ball you will see the arm action I speak of. The arm should be bent throughout the shoulder rotation until starting to release the ball as we then want extension and snap as we bring our chest point at the catcher.
Look at Samardzija in slow motion. Watch at the 18s point in that video of his foot plant and where his throwing hand still is. Slightly below the shoulder moving upward. Look at front side (the direction side) and his shoulder to elbow height.
Look at Storen in slow motion. At 21s in this video is just as he is planting his front foot. Throwing hand still below shoulder moving upward. Storen has a high front side arm, but even he is close to horizontal.
Look at Daniel Bard @ youtube (watch?v=0-vOceb1Cho) and go to the 2:00 min mark just before foot plant.
You son is pulling his front side down and not driving his chest forward. If your front shoulder (direction) is going down, what does your rear should (power) have to be doing? Going up.
Tough to see all that is occurring from a video, but I hope this helps a bit.
Best to you and your son