Isnt there another 'male' you could ask to go with you to make it look like hes your new bloke?
i dont really like playing these type of games, and what youve gotta ask yourself is WHY is he your EX? but if you just want to guage a reaction - then walking in with another bloke might just give you a clue as to wether its worth persuing or not? Bear in mind he might be there with another girl (with the same plan in mind you never know)
My best advice would be to just go with a load of great mates and have a ball, and dont even think 'MEN' ??
hope you have a good time whatever you decide x