Agree he needs work on his glove side. To start, he needs to get into an equal and opposite position with his front arm. Right now, it looks like his front arm is extended straight out in front (the ol’ “point the glove at the target” teach). Make the fix in the glove arm - leave the throwing arm alone. The fix is to put the same bend in the glove-side elbow as in the throwing-side elbow.
Then, from the equal and opposite position, he needs to stbilize the glove somewhere in front of his torso instead of out to the side. Have him visualize a wall in front of him and that he’s going to stick his glove on the wall in front of him and keep it there.
Do not have him pull the glove back.