GETTING THROWING MUSCLE MEMORY BACK

I have coached with my son for many years. He is now in 10th grade and starting SS, CF and Pitcher for his varsity team. He easily has the strongest arm on the team.
Through the years he has never had arm problems. I credit that with two things, lots of backyard throwing and good mechanics. From day one he started throwing by warming up at a close distance (20ft or so) and ended up with long toss. We measured long toss for him as normal center field depth to 3rd base. He has always thrown at a target, most of the time my glove. We never just went out and tossed the ball around unless he was working on a particular pitch. Even then he was aware of location and rotation of the ball.

I've read in many books and been taught that "over the top" is not correct and will cause arm problems in the shoulder. Here is what I’ve taught my son to do as a position player.
The grip on the ball should be like a four seam fastball. I have always taught my son to keep is elbow higher than his shoulder. This takes strain off the elbow and shoulder. His arm should travel a path from 11 o'clock to 5 o'clock. At the start of the throw his body should be sideways and inline with his target. He should use his glove arm elbow as a site to the target. His throwing hand should be facing away from him; He should reach back as far as is comfortable. His fingers should be behind the ball, pointing to the sky, while his thumb is underneath the ball. If he looks back he should be able to see the back of his hand and his fingers and thumb in the right position. As he comes trough, his hand will naturally turn around/over. He should end up with the back of his shoulder facing the target, letting arm finish up naturally. In other words, he should not try to slow his arm down, but let it follow through (to about the 5 o'clock position). Trying to slow his arm down will cause strain to the shoulder and elbow too. The idea is to get a true, backwards spin on the ball. The backwards spin will make the ball travel straight and true.
Kids have a tendency to have a "lazy wrist". A lazy wrist is when they come from the 11 o'clock position and let their hand fall over (away from their body) instead of keeping it up and down as they come through. This will cause a sidespin on the ball, similar to throwing side arm.
I picked up a training tip from a coach that works well for us. Paint a 1/2 inch black stripe all the way around, and across the four seams of a baseball. If the ball is held in a "four seam fastball grip” the stripe should fall between the two fingers, and the on the thumb underneath. If the ball is thrown properly, you should see the stripe coming at you in a vertical position, with little to no wobble. If it is spinning the hand was not in a vertical position when the ball was released.