Sit him down and look at videos of hitters on this site and others (maybe Baseball Fever:
http://www.baseball-fever.com/...5c29c69a0c4b49&f=53).
Take notice how the body moves when loading and unloading. In that process, you will see the shoulders pinching closed as the lower body is moving forward. That allows them to behind "behind" the hips in the loading and unloading process.
I am willing to bet you that your son's shoulders open at the same time as his hips because they are loading and unloading together.
Look at different player's swings, but notice the similarities in the process. That will educate you on things to work on.