Please help me deal with mom issues

Mom issues…

I have a 10 almost 11 year old son. My son has always had an amazing relationship with my mother. Her and I on the other hand have always had issues. She has taken me to court four times since having my son trying to have him taken from me for multiple reasons. The first time being she kicked me out and kept all my belongings then took me to court saying I didn’t have a crib for him. The judge finally got fed up and told her enough is enough she is no longer allowed to take me to her court room anymore. I have looked past almost everything my mother has done to me for the sake of my child and her relationship. Well about four years ago things got rocky when she got in a new relationship with a much younger man. He was great at first and over time he has became every abusive verbally and physically. My son has been at their house and witnessed these actions. I stopped her from seeing him after finding out due to that being an unsafe Environment for a child. Since I stopped the visits my son has became very depressed and tonight asked me about seeing her. I explained to him that he can’t and why I don’t feel it’s safe for him to be there. He just started crying. It breaks my heart to have to keep him away from his grandma but I know it’s not safe for him there. I have asked get numerous times about seeing him alone just my son and her outside of the home and she refuses unless her significant other can come. I need advice on how to handle this situation with him and make him feel more comfortable with my decision.