I need some advice .... so my SO has a son and after 6 years he found out it wasn't his son . I told…

I need some advice .... so my SO has a son and after 6 years he found out it wasn't his son . I told him not to cut the little boy off , well that was until the son's mom kept popping up at my house and job , and then I caught them two in bed together, and me and her got into a physical altercation after she was saying very disrespectful things about my son . My and my SO stopped contacting eachother until I found out I was pregnant again and we decided to make it work , and he asked me if he could start seeing the like boy again. I told him yeah but no contact with his mother , yet he confessed to me today that he has been having conversations with her and idk what to do . I don't want him to cut the little boy off but I don't trust my SO and the boys mom , so what should I do ?