I don't want my baby around his father's gf....

My son's father is currently in a very toxic relationship and I don't want my son exposed to it. I got pregnant last May and his father got with his gf in July.  Since they have been together from what his father has told me, they have had the police called on them multiple times for fighting, his gf in an alcoholic, she beats him often, and she threatens to kick him out weekly. This is not a woman I would ever want around my child. She is very unstable and it makes me EXTREMELY uncomfortable. What can I do to prevent my son from being around this woman?  I've voiced my concerns with his father and he just says it's not up to me who is around our son during his time with him. I know in my home state that he's not allowed to have my son over night until he's 3.  I would appreciate any advice or help from anyone.