>>> My son is 13 and bats right-handed. He has been told that his left shoulder is coming through too soon and he is losing some power; also making it hard for him to hit to right field and to pick up low-outside pitches. Any ideas for how I can help him improve his timing and coordinate hips/shoulders/hands to deliver all of that energy at ball contact instead of spending it too soon? He seems like he's rotating his shoulders too soon... <<<
Welcome to the site. - If I am reading your post correctly, your son is experiencing "bat drag" in his swing. In other words, when his shoulders have fully rotated (facing the pitcher), his bat is still some distance from making contact. By the time the bat-head does come through, his hands are arcing to far away from the contact zone to make solid contact -- especially on out-side pitches
If this is the case, I would suggest you are placing the blame in the wrong area. Rotating his shoulders too soon is not the problem. In fact, it is the initiation of shoulder rotation that should accelerate the hands to the contact zone - not the arms. "Keep your shoulder in-there" as the batter initiates his swing, is a linear mechanic concept you will not find in the best hitters.
As the video clips below illustrate, the best hitter's shoulders start rotation to initiate their swings. Therefore, the key to a good swing is the batter's upper-body mechanics that keeps the angular acceleration of his bat in sync with his rotating shoulders. -- This is the area I would suggest you and his coaches concentrate on.
Rose hip/shoulder comparison
Overhead view - Rotational Principles
Bonds shoulder/bat rotation