Your photography is amazing. Your DS seems to get most of his power from throwing hard. He uses brute force and arm speed. He has 5+ MPH available right now with improved form. Better, the speed requires less effort. If he can learn to whip, a whole new world will open up to him.
Think about hitters. Do they swing a bat with straight arms and the bat in line with the arms? What about a soccer player? Is the ball struck with a straight leg and the toe flipped at the last moment? How about a golfer? Are the wrists locked with no pivot point? Last one: How about the MLB pitcher? Does he throw with a straight arm and flip the ball to over 90 MPH with a good wrist flip at the top of the circle?
A quick look at slow motion video will tell you that none of the above happens. So why do we tell our kids to speed up that arm circle and flip the wrist, or even the elbow/lower arm abruptly at release? Slow motion video of the highest level pitchers will reveal that none of that stuff occurs, but it still gets taught--over and over...
Simply, the best pitchers whip the ball. Watch the below slow motion videos of some pitching greats and take note of the flexed elbow going into release. Also notice how the upper arm pauses at the torso and the lower arm accelerates. The ball leaves the hand at a much greater speed than the arm was traveling around the circle. If you add forearm brush to the mix, the speed and consistency goes through the roof.
Yukiko Ueno: