Roger - Thanks for the detailed analysis and advice. He threw a bullpen today (before I saw Roger's post). His first few pitches I could tell that he was still leaning back, so I told him to focus on staying upright and balanced, and keeping his head level and pointed toward the plate. The rest of his pitches were down in the zone for "good" strikes, as opposed to tending toward being high and arm side. Definitely progress. We'll keep working on incorporating the advice in these posts little by little as he becomes comfortable with each new tweak. I'll post some more video in a few months.
Thanks again to everyone for the great advice.