The insider added: "Sam is obviously devastated. This is the most significant relationship he has ever had. Thankfully he is on tour across the US at the moment which has been a welcome distraction."
The news comes after the singer was spotted enjoying a spontaneous night out at Massachusetts club Candibar on Sunday with a small group of friends.
An eye witness said: "I don't believe he was drinking. I don't think people recognised him. I would have thought people would swarm him, but it didn't happen.
"The people he was with were trying to prevent people from taking photos/videos."
Brandon was not with Sam at the time, as he was celebrating Gay Pride in New York with his co-stars Tommy Dorfman and Kate Walsh.
The split comes just months after the singer claimed he was excited to finally pen some "happy love songs" after finding love with the actor.
Speaking in January, he said: "I'm in a relationship right now and for the first time, I think I deserve to be happy. I'm actually asking myself if I'm going to be writing some happy love songs soon."