I think his hips are late to rotate and are coming through in one big rotation with his upper body. Look at the difference between Greinke when his arm is fully cocked and when your player's arm is. From what I can tell, Taber's quad is still facing the camera, and I still see the front of his shin. Greinke starts his move to the plate with a his back hip firing, so when his arm is cocked and his shoulders are still back his back hips has already done a lot of rotating.
I think your player is trying to do it all with his arm, evident in the difference between he and Greinke at this high arm cock position. Your player's chest is really up at the sky almost and his elbows are pulled way down and back. Greinke instead has level shoulders and fairly quiet upper half, probably because he's letting his hips and lower half do the heavy lifting.
For reference, I'm looking at 0:44 in the Taber video and 0:28 in the Greinke video. Hope that makes sense. Have Taber focus on his back hip starting the rotation to the plate. The thought should be not to just open the front hip, but to drive the back hip through.
Definitely curious what others see though.