New Mechanic = soreness ??

I don't quite understand your description of the changes but if any that take place prior to release alter his posture or timing then that would be enough to put more stress on the arm.

Maybe you can clarify the parts about "kicking his post leg" and "ending up with his arm and shoulder outside his hip"?

He was finishing with his head up and arm basically going to his right knee (LHP) He was asked to more or less bow his head, allowing arm and shoulder to finish outside his right hip and at the same time kick his left leg after release to completely follow thru. Kick isn't quite the right wording buy I can't think of a way to better describe it.

I wouldn't really have a problem with the kick, the more you can get your lower body into your motion, the better. However, the head drop and shoulder finish advise could be causing him to open up his front shoulder too quickly and therefore dragging his arm. This will put a lot more strain on the shoulder and elbow. Shoulder should stay closed as long as possible to allow the arm to keep up with the body, throwing as a unit. Don't know for sure without seeing him, but this is a possible cause.

Thanks for the clarification. Here's my take.

Tucking the head: My first thought here is that tucking the head won't do anything and that a bend at the waist might be a better tactic. I teach keeping the head upright throughout the delivery. It might drop a bit near the end of the delivery as a result of the forces of throwing the ball hard but I don't believe it should intentionally be tucked. Depending on when this "tuck" occurs, it could certainly lead to other issues as bballman mentioned. Relating this to pain or possible injury, tucking the head cold cause a late posture change. A posture change pulls the head and spine offline from the target and will have the effect of pulling back and raising up the release point possibly causing the pitcher to try to compensate with the arm.

Arm and shoulder finish: I believe that where the arm finishes is largely a byproduct of arm slot. For example, a high arm slot finishes low while a side-arm slot finishes waist high. Trying to force a particular finish that doesn't match the arm slot is a recipe for injury. Now, if the follow-through is being abbreviated either in the arm path or shoulder rotation, then that should be corrected to allow deceleration to occur over a longer distance instead of too abruptly.

Leg kick: As with the arm finish, the back foot comes around in a path that mirrors the arm slot. And that makes sense - all the rotational force that is generated in a the plane of rotation creates inertia that pulls the back foot around in that same plane. So, trying to force the back foot to take a different path will increase the chances for pain and injury.

This is all IMHO, of course. Ultimately, this is difficult to discuss without seeing some video.

I appreciate those taking the time to reply. I hadn't heard the tuck the head advice before but I do believe the intent is in fact to get more bend at the waist. Once his tryouts are finished I will try to get him on video. Thanks again.