Well I really doubt that he can throw mid 70’s with those kind of mechanics, his lower body is virtually non-existent when it comes to creating velocity. Also, It helps more if you can do a full speed video because he might be changing things to slow down for 3/4 speed.
But a couple things I noticed…
1-Like I previously said, he has no drive with his back leg, and I can’t quite tell in this video, but I think his hips might be opening too early. Try and get him to drive atleast 80% of his total height, so if hes 5’, aim for a 4’ stride.
2-His elbows are at about a 70 degree angle in relation to his shoulders when he begins to move his arm forward. You want the elbows to be atleast parralell to the shoulders, otherwise you put extra strain on the shoulder.