Slow Arm

My 17 Year old RHP, 6-6 210, has been said by several coaches that he has a slow arm. Any thoughts on ways to speed him up? 77-82 mph

Just looking real quick it looks like he is giving up his shoulder rotation early. So he appears to be pushing the ball. He does not have very good external rotation (ER).

Put up a normal speed clip (preferably a GIF).

6’6 215