Pete Wentz had a blast when he deejayed Office Max's Green Tie Ball (benefiting Chicago Gateway Green) on Saturday night in Chicago.
Although he was there to spin tunes, Wentz brought TopShop clad girlfriend Meagan Camper to the black tie event.
"Pete and Meagan stayed within arms length of each other the whole night," an onlooker tells PEOPLE. "They were super sweet."
"She joined him to watch his set from the back of the stage and danced to his set list," the source adds.
Dressed in a dark green SuitSupply suit and bow tie, Wentz played songs from bands including Gym Class Heroes and Fun.
"He kept jumping on the mic to tell the crowd how proud he was to be from Chicago," the source adds.
Us savages are about to crash this civilized party. Get ready.— Pete Wentz(@petewentz) September 16, 2012
– Lesley Messer