Pi Kappa Winning!
TMZ spoke with the president of the frat, who tells us some of his brothers approached Charlie on campus on the day of the show -- and they gave him their frat's shirt.
Charlie ended up wearing the shirt during his show -- and gave the guys a shout out from the stage.
After the show, they got to hang out with Charlie backstage -- and get this, they even offered him an honorary alumni spot in their fraternity. He thanked them for the offer, but hasn't taken them up on it thus far..
Late nights, hot girls and partying ... sounds right up his alley