He was mentioned after and along with 19 other Gladers have been confirmed to have survied the trials.
FilmBilly is seen among the surviving escapees at the end of the first movie, The Maze Runner. It was confirmed by one of the film producers that Billy was killed off-screen by the Cranks in the beginning of the second film, when the teens are leaving the helicopter.
In Maze Runner: The Death Cure the name Bill is briefly seen on the side of the Memorial rock. It's possible this is in reference to Billy.