In the book, Simon gets a "gay vibe" from Cal, and for a while really believes it was him. After sending an email describing some things he and Cal have done together and Blue replying no to most of them, he realizes it wasn't him. Cal later comes out as bisexual.
Cal is polite and nice to Simon throughout the movie, and after finding out it wasn't Bram or Lyles, he settles on the fact that its Cal based on the "family trip to the middle of nowhere" he talked about during drama rehearsal and emails. After Simon's emails are leaked by Martin, he asks Cal if he is Blue, and Cal denies, but offers to listen if he needs someone to talk to.