First of all, I would mostly try to have him work on throwing longer distances with very little time pitching until closer to the season. He mostly needs become as athletic as possible at his age (like everybody else). Small doses of long toss, some time throwing football, anything to keep him moving. His patterns are a little out of sync but mostly very good for his age and size. There is a big yard outside that cage. He should be trying to throw all the way across it.
Not sure how he compares in size to his peers, but he looks slight. His velocity will improve as he gets bigger and as his overall athleticism improves.
His arm path is very short so he gets his arm up to the high cock position before his front foot is down. His arm has to wait on his body and this slows arm speed.
I think longer throwing distance with emphasis on getting his glove up to the bill of his cap will help some. Opposite and equal with the arms may be a cue that can get him to engage his front side more.
While normally I agree that pitching from the stretch is better, most of your fellow's deficits will be better served by having him throw with movement. Shuffle throws and step behinds and step overs will help him sequence better and also improve his field throwing. He needs to become a more fluid and better sequenced thrower before he becomes a pitcher.
He does lower his elbow. This is a position that some very high level pitchers get into. (see Maddux) He does not push the ball. If you step through the videos frame by frame, you can see that he has very good forearm layback or external rotation. Do not worry about that part.
In short he needs to have his front foot down before he starts to throw the ball. I think the best way there is to have him throw with movement and then try to take that to the mound after his pattern is more developed.