Hi Mark,
If you haven't already seed this, check it out:
http://www.youtube.com/watch?v=zu4vLOEthM4
I'm with Turn 22 and feel the recommendation of the "Hershiser dill" would be an excellent drill for your son to work on. Note the person teaching this drill in this video is Tom House, a well know pitching coach. It was House that showed me this drill many years ago and I had my son practice this when he was 14 yrs old and it helped his pitching in more ways than one (increasing his velocity and constancy to name a couple). So, yeah, I HIGHLY recommend it.