This sounds like a tough situation to be in! What makes you think his parents are being violent to him? Is there physical evidence?
My first suggestion would be to try and talk to him about it. If what you suspect is true, then it's going to be a delicate issue, so approach with caution!
There's also a chance that, even if he is being abused, that he may not want to make it public. There are many complex reasons for this.
However, if he is in danger, then I'd say taking action on his behalf is justified. It should be seen as a last resort, but if his parents are putting him in danger of serious harm or even worse... then speaking to a teacher, social services or even the police would seem appropriate to me.