I think that his step behind will allow for better momentum development. I would keep that.
Yes he could lead with the hip better.
He looks to be too careful with his delivery. To slow and controlled. He should try and speed up his delivery. His loading and unloading should be as fluid as possible. He needs to start thinking about throwing the $%#@ out of the ball as soon as he starts his leg lift.
Right now it looks like the last 20% of his delivery he starts to think about that.
It will take time, but keep at it. 77 is pretty good for a kid his age. Try doing more long toss withg him and incorporate those "long toss mechanics" when he's on the bump.