is it the 3-pickup version? i assumed so given your comments in bandit's recent thread, but here you've made it sound like it's just got the two pickups.
i'm pretty sure that on a 3-pickup black beauty the middle position is the middle pickup only. i could easily be wrong, but i remember reading up on 3-pickup gibsons and the epi SG custom being like that. try replacing the switch, because that's annoying no matter what. if you still have problem, try and favour the other pickups (i know that's not much help, sorry). it might be normal, a middle position humbucker might be quieter than either the neck or bridge when selected alone.
also, not to state the obvious but just in case, see if he's lowered the middle pickup a lot in order to give himself some picking room. i've found the 3-humbuckers make for some crowded playing (pick keeps hitting it, etc).
if this is a recent problem he never had before without changing anything on the guitar, i'd suggest testing the pickups with a multimeter to see if anything's wrong with either of them.
hope it works out before the gig!
