Really awful this has happened. Your poor brother.
Agree the immediate priority is that he doesn't have to go back to that house to stay with that man.
For the longer term, years ago a boyfriend of mine confided in me that he had been sexually assaulted by another man. So I know that feeling of explosion and helpless frustration that you cannot tell other people, people who could help protect and support your brother. I called survivorsuk, as detailed above, and they were very helpful.
Their advice was, be supportive, encourage him to report it but don't force him into anything he doesn't want to do (even by arguing forcibly that he do so- he already feels forced to do things against his will, traumatised and violated, and pushing him too hard could bring that back and cause mental/emotional collapse), be there for him, love him, don't waver in the love and support. They were very clear on not forcing the issue, even if it meant telling him to phone them himself.
They were also very clear that I could call them back anytime if I needed support and help in supporting my boyfriend, and if I felt like I was about to explode they would be there to listen and support me. They recognised that helping my boyfriend get through this would be an emotional strain on me too, and they would be prepared to support me emotionally if my boyfriend didn't want their support directly but through me.
HTH. Wishing you and brother strength to get through this. 