My moms partner has aspergers and is abusive, HELP!?

My mom isn't in a relationship, but she considers this man her partner, he has been living with us for ten years and my mom is considered his carer by the government because of his aspergers. He used to have fits occasionally but they have gotten so much worse, to the point now that a day doesn't go by where he doesn't get angry and hit something or one of us. He has hurt my mother several times (she never says he does but I can see the evidence) and he has threatened myself and my little sister. I know a lot of people would say 'just kick him out', but this is a catch-22 situation because my mother has OCD to a point that she cannot be alone at all; someone must be with her 24/7 and that is usually her 'partner'. She can't get rid of him because she can't live alone, and I am leaving for university in a few months time. I'm worried about what he will do to my mother and my sister, he is abusive and is always swearing at us and making rude gestures etc and threatening to 'show us who's stronger'. What can I do?