Really solid mechanics in my eyes. He gets a lot of extension and really employs his back side will when releasing the ball.
The only thing that looks a little constrained is his upper body honestly. Like everything is a little “bunched up.” The throwing arm cocks up with the baseball right behind Kai’s head. Seems like that might add a little unneeded torque to the throwing arm’s movement. Not sure on this one - not a major thing but curious what others think.
How did I miss this thread? I agree with @sidewinder34 on the upper half. It does seem the forearm is angled beyond the 90 degree line toward the head and that the ball is significantly inside the elbow as the arm comes forward. He does seem to get it online by the time his shoulders are square. The only other thing I see is the elbow getting ahead of the torso. Those two things may be cause for some elbow concern depending upon how hard he throws.
Fairly decent just your elbows pushing put in front of your face creating more valgus torque on the elbow.