We break a lot of stuff down including the pitching motion. Give this a try. Have him stand as if in the stretch. Pretend he has already made his stride and his foot has landed. Then, have him point his lead elbow at the target and have his throwing arm in a 90 degree straight with the the arm from arm pit to elbow running parallel to the ground. Now, have some type of obstacle to throw over. We use a pitching L-Screen but prop it up higher to raise the bar the pitcher has to throw over. Stand behind your son and toss a ball to his hand. Have him catch that ball and then try to throw over that obstacle. I hope I've understood your problem and that this helps.