Have him try this. Get to “equal and opposite” (stride has already happened, glove hand and throwing hand are in equal and opposite positions). Have him rotate his hips, but stay in the equal and opposite position. This is the starting point. Weight should be evenly balanced between front and back foot.
Have him throw from his low elbow position. He may try to tell you that he doesn’t throw like that normally. Have video to prove he does!
Have him reset to starting position, raise his elbow to the correct position and have him throw again. Ask which one felt like he threw harder. Hopefully, he picks the second one!
Have him throw 5-10 with the correct elbow level and then start working back through the pitching chain making sure his elbow is coming through correctly. Once you come to a spot in the chain where he is having trouble adjusting - stay there and work it out. It might take 10-20 pitches or it may take 100 or more. The key is - don’t push him beyond what he can do. He’s got to be able to feel it throughout the entire chain or it’s all a waste of time.