I agree on the lower half as well, a bit out front. Simply squaring the (right) back foot will aid in keeping the knee inside the ankle and keeping the weight back, known as negative movement.
He tends to "pull off" the ball, a by product of standing up in the swing (very common in younger players with muscular imbalances, core strength in this case) . Staying through the ball will create drive by connecting the lower half, known as positive movement.
Loosening the grip,especially the top hand, will in help increase quickness as well.
Swings are built and you work on "holes" in the swing just as much as the strengths, IMO at some point the athlete simply takes over. Sometimes you just have to slow it down, hitting ball after ball has very little positive impact. Every swing has a purpose and approach.
Fine looking kid, good luck this season!