You need to talk to him. I'm sure he has an idea of what your REAL financial situation is if your going to be his bestman. I can understand him expecting one of the big items(plane ticket/hotel/whatever) to be paid for him between you and the other guys but for him to want to go all out on a trip to Vegas like that and expect you to foot the bill totally is crazy talk... especially if he is the big money guy between the 2 of you. You need to get a general plan and lay it out for him on the options he has available.

There is no reason you should be forking out even a quarter as much money on his bachelor party as he is on his own wedding.