When he sits with two seniors, Sam (Emma Watson) and her stepbrother Patrick (Ezra Miller), at a football game, they invite him to join them to several social activities. At a party, Charlie unwittingly eats a cannabis brownie, gets high and discloses to Sam that the year before, his best friend committed suicide. He also walks in on Patrick and Brad (Johnny Simmons), a popular athlete, kissing. Sam realizes that Charlie has no other friends so she and Patrick make a special effort to bring Charlie into their group. Sam needs to improve her SAT scores to be accepted to Pennsylvania State University, so Charlie offers to tutor her. On the way home from the party, when the three hear a song with which they are unfamiliar, Sam instructs Patrick to drive through a tunnel so she can stand up in the back of the pickup while the music blasts.

Based on this passage, who does patrick date in perks of being a wallflower? Let's give stream of consciousness first:
The relevant information to answer the above question is: He also walks in on Patrick and Brad (Johnny Simmons), a popular athlete, kissing.
So, the final answer is Brad ( Johnny Simmons ).