I need some quick advice!!!!!!?

I think you have to talk to his mother before you talk to the police. Tell her what your stepson said and that he seems frightened. She may be upset with you but not as upset as if you went to child protection without even talking to her. If she does nothing and you think some abusive behaviour is continuing then try calling the authorities. I really hope it's not what you think, but good on you for caring and good luck.