Has anyone looked at his stance when catching?
One impacting factor could be that he is not in an optimal position when catching.
You need to relate doing mobility work to improved performance.
Let him know how improving these areas will allow him to use more of his potential as an athlete.
I would want to see his position when he is catching first.
This could show alot.
If less than optimal position fix the position first.
If there are restrictions to getting into an optimal position for catching you need to mobilize at the position of restriction.
What is the cause of the restrictions and go after the place where you will see the biggest improvement in positioning first.

Improved mobility= taking a hand brake off on your ability to perform at your true potential as an athlete.
Improved mobility= improved performance