Don't try to tackle too much at once. It's too hard to do for one thing and it can be discouraging to a youngster. Make sure he knows the things he's doing right; staying on top, keeping his arm up, throwing strikes, etc.
The glove hand is important too, and curling it in is important to correct as soon as possible. However, again, I would start with one thing at a time!
Best wishes, and keep us informed of progress!!