Aw, im sorry to hear about that
I agree, the best thing would be just to make sure your mum knows what is happening.
The only other thing i could suggest is to sit down and talk to her properly. I mean, dont just tell her what her bf does, talk to her about your feelings, how it makes you feel and how you understand that her situation is difficult etc. I know this can be really difficult to do but at least that way she may get more of an idea about where you're coming from, as opposed to just seeing her own point of view. It will also show her your maturity and she may begin to realise that she hasnt been seeing the bigger pic, if that makes sense?
Im guessing that you love your mum and you want to stay with her? So in my own experience, its her choice who she lets in....
Im in a similar situation to you but with my real dad. I dont know what else to do except involve the police and social work, which i am not prepared and cannot afford to do. Im really sorry i couldnt be of more help but i hope things work out for you soon. Let us know how it goes.