What pickup does he have for it, out of interest?
The best solutions I've come across usually involve a piezo under the bridge combined with a small condensor microphone. Not cheap, and requires some pretty nifty work on an EQ if the bassist wants rock band level monitors!

Cheaper pickups are usually just the piezo, which IMO almost completely defeats the object of having a double bass - you get the notes being played and a big bassy woofle, but lose a lot of the warmth and quality of it being an acoustic instrument.

YMMV, and I have very geekishly high standards when it comes to this sort of thing, I mix FOH for quite a few folk & acoustic acts!
(Son of Sam-i-am We're all as mad as hatters here, Mon 15 Dec 2008, 13:47, Reply)