Tell him that on his stag do everyone in your group is going to wear stockings, suspenders and a g-string under their trousers. Tell him its for a gag, so you can flash chicks etc. Obviously, none of you do it but him, and when you see him chatting to a nice girl, you all jump him and pull his trousers down in front of her. Thus begins various rounds of ' pervert ! ' and ' freak ! '. When he tries to assure the girl that it was a joke, everyone is wearing them, you all drop your trousers at the same time to reveal normal boxer shorts.