I can only express what I have experience with correcting something similar in my youngest son (9yo). He had a similar shot put type motion with a lower elbow. I came to realize this was caused by the teaching he had received that involved the Down, Back and Up into 90% position while still fully closed with the shoulders. You have seen it, the swinging motion right into ball up pointing at second. Then, on rotation he would pull the elbow back down, probably his body over-correcting, then his elbow would lead turning it into a push.
What corrected this was realizing that the arm action needs to be down, till the hand breaks the body line, then the elbow pulls back into scap load (not up into inverted W but back behind the body, but like you see in the maddux pic), this puts the wrist only slightly above the elbow, the arm doesn’t go into the 90% until the start of rotation, think when the glove side goes from thumb down to thumb up and the shoulders rotate, the throwing arm rotates up into the 90% cocked position…
Removing the high early arm cocking corrected the elbow drop and push, the ball no longer pops up out of his hand at release and he now has much better accuracy and he throws harder and more naturally. Before everything upper body was too early and it looked really strange.
The issue with many still pictures is understanding the actual timing of the arm cock, you always see the arm in 90% at foot down, but to me it’s really at foot plant (the point where the front foot becomes load bearing and stops movement (bears up against momentum), this is when the rotation of the shoulders has just started. In Mr. Ellis’s photo above the glove has already rotated up and you can tell the shoulder is already starting rotation… In Lefty’s pic of Maddux the glove has started turning up and his throwing hand is in the act of arm cocking. My point is that if the glove side is still pointing down and the arm is fully cocked, that’s too early and I found leads to the issue you have. I think this happens because your arm cocked so early has nothing left and the body has to create more power by either pushing the elbow down and out, or in some cases over rotating.
Again this is only what I found to be the case as I started paying closer attention to my own throwing motion and comparing it to my son’s and I don’t profess to be an expert at all.
Try an experiment, put your son in almost the same post stride position as the Maddux picture above, glove pointing down and throwing arm with elbow pulled back behing back and wrist relaxed and slighly above elbow. have him flip the glove thumb up and at same time rotate his throwing arm from maybe just a little lower than the pic into 90% and at the time of natural rotation and throw. Remember not to swing the arm up towards the back but to rotate the elbow up into an L Make sense?