I'll start but I'm by no means any sort of expert.
He needs to lengthen his stride to about 80% of his height (just a rough number but a good start).
He needs to keep his glove arm more in front of his body. He let's it flop to the outside.
He needs to finish with his R hand near his left knee.
He needs to use his legs.
Andrew looks for all the world like a twin to a kid on my son's TB team.