I'm sorry you are going through this. Your first priority, I think, needs to be your son's and your own safety. You have to protect you son.
I agree with others, you should ask your son privately if this has ever happened before when you are not there. YOu may find out your son was scarred and didn't know who to turn to. If your husband refuses to get help or continues to put you and your family in danger-you need to leave. Your priority needs to be your son's safety.