Guys were mini skirts in Cancun. I know it sounds crazy but something about Cancun club culture. You wouldn't want to be left out so be sure and pack a mini skirt or two.
You know I thought about my previous response and decided that I was infact being a JACKA$$. Seriously man you can wear what ya want alot of guys were shorts or whatever they wore on the beach earlier that day. Some guys wear pants and button ups (though very few) but it gets hot in the clubs so this would be ill advised. If your inclined to wear a Kilt I promise not to laugh. But what's really in style for 2008 is a TOGA!!!
I know I saw several guys out at the bars/clubs wearing board shorts and t-shirts so I would think that's fine. Maybe check w/ some of the other Cancun Vets around here as I've only been once and didn't get to hit all the clubs. Maybe there are clubs with dress codes but I don't know.
in cancun there really isn't a dress code out in the clubs like hats/durags/tank-tops ect....but keep in mind you gotta dress as light as possible and i would dress as casual as possible too,cause it does got hot in the club both physically and vision as well....