wearing a retro suit and a prosthetic nose for his latest role as Leonard Bernstein, for the upcoming Netflix film “Maestro.”Now, a former protege and friend of the music legend is revealing private stories that the Oscar-nominated actor has likely never heard about the composer of “West Side Story” — including how the married Bernstein slept with his protégé’s boyfriend.Conductor Mark Janas, 70, was friends with Bernstein beginning in the mid 1970s until the composer’s death in 1990, and last month he managed to get a letter to Cooper, through the actor’s, Hollywood agent detailing some of his stories of “the great maestro,” which he shared exclusively with The Post. Cooper, 47, is directing and starring in the movie, scheduled for release next year.