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he will probably find the easiest way to learn the CC is to just play it. when forced to "adjust" students seem to pick it up very quickly. He should have little or no trouble after about a week or two, if he just plays CC.
best of luck to him

on a side note... a way that i found worked really well (fast) was to do a lot of sight reading.

Depends on what going on. Is he currently preparing for a big performance?
If so, stick with the current horn until it's over.

When there are no pressing, near term (less than a month) playing requirement, then put the old horn away, and play the devil out of the new one. Play old, familiar peices as well as any new material that need to be prepared.

After a few months on the new horn, it might be good to start playing the old one again to keep both fingerings up to speed.

Just like a foreign language, the only way to really learn it to remove the old crutch. For a language, you take a trip where it the native tounge, and don't speak anything else for a while. (assuming you know the basics already, on a horn they are simpler)You'll stumble a lot, but soon you do fine. Then you can do both, as long as you don't let one go unused for too long.

-Thomas "whishing I had made a trip to Germany after 6 years of german study"

I'm a college freshman and I made the switch a few months ago in the first semester. I couldn't quit Bb tuba totally b/c of marching band but, I did all of my serious playing on CC. I think that if your son just practices on CC and just leaves Bb for marching band he'll be okay. I had no problems with the switch...in fact I played in a recital the month after the switch and now I'm working on a concerto for April.