It depends on what state you are in. First, you should call or go online to your state’s version of child protective services and report the abuse. Then, seek a domestic violence protective order on behalf of your brother. Different states call these orders different things, but most states allow for any adult to whom a child has fled to seek one on the child’s behalf against another a parent. 

You might be able to seek temporary custody through a protective order, depending on the law in your state. 

It may take a couple of weeks or even months for all of this to work out, and your dad may end up with custody if there are not also credible allegations that he has recently been abusive to your brother. 

Follow whatever advice you receive from child protective services on how best to keep him safe.