How to help my son through a best friend break up

My son doesn't have a whole lot of friends. Honestly he only had one and his friend just ended their friendship two days ago. My son is openly gay. He came out last year. His best friend was fine with it. Until his best friend broke up with his girlfriend and she started a rumor that my son and his friend hooked up and had sex in the school behind the bleachers during an assembly. He's been bullied and even got called into the office but since there's no proof that happened they can't do anything. My son swore that never happened and his friend ended the friendship to prove he isn't gay. My son was supposed to celebrate turning 15 next week and him and his best friend were gonna see a movie and have a sleepover but now he doesn't want to do anything. It's so sad seeing him like this.