You might have a great wool suit, but a nice cotton suit will save your life come the warmer months. Besides, this is a wedding, not a funeral (well it might be for your buddy's days as a player), so there's no reason to wear a black suit. Go for a dark navy or gray. You don't have to go completely unstructured, since a little padding or canvassing helps the suit hold its shape. J. Crew's Ludlow suit is a nice, slim fit that looks great on a lot of guys, and they've recently made it in this summer-appropriate Italian cotton piqué fabric to keep you cool while looking it.
Ludlow Double-Vented Suit Jacket in Italian Cotton Piqué
Buy It Now at J. Crew, $298
Ludlow Italian Cotton Piqué Suit Pants
Buy It Now at J. Crew, $158