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Has the left shoulder experienced trauma?
Is he continuing to work on the tspine?
Have you watched the webinar about the shoulder?
The Athletic Shoulder: Anatomy, Physiology & Function
Understanding how the shoulder works is beneficial
Jill Miller Fixes Your T-Spine and Crappy Breathing Mechanics
Jill Miller has some great videos here that are a great additional reference.
There is a link for a free quick fix here you can access others for a small fee.
The shoulder, upper back, and neck videos may hit other impacting areas.