My bf's parents are abusive and really need help on what to do about it!!?

My bf's parents are abusive. They lock him in his room with no food for days. They hit him and scream at him and ground him for no particular reason. He has a mom and a step dad and five siblings. They are not abusive to his siblings, and he feels like nobody understands. He is the oldest and 15 years old. His parents don't know that we are together. They expect him to be perfect. He has to get all as in school or they will lock him in his room and call him names. He fells like he is completely worthless and is depressed every time I call him. He even asked me why I like him! I don't really know his parents that well, but have met them once or twice. They did seem nice, but why are they so cruel to him? And what can I do to help him and stop this? I'm the only person he'll talk to, and nobody else knows. He's tried to get away before, but only ends up getting into HUGE trouble. His step dad is really huge, and he uses that as a thing to, like, control him.