Boyfriend

So My boyfriend has a ex who he was engaged to and has a child with who is harassing him. It is getting really bad to the point that he hides away in different room to try and calm down. He loves his son but the mother is being a real b****. He dose not want to lose his son but she just won’t stop. She told him that she was going to make stuff up to the cops to get him arrested if he didn’t get back together with her. I am trying to be a supportive gf but I don’t know what to do in this situation. Every time he meets her to see his son; he has a voice recorder on so he can prove nothing happened if she try’s to say it did. What would you guys do in this situation? I just want to be there for him,but I don’t know how.