My partner (R) and I just had a child and have been discussing marriage. Overall we are happy and doing well. There is one really big issue that has come up though. I don't trust him or the mother of his previous child (age 7). 
 

When R and I first started dating, she was living in NY and we are in KS. When she found out that I was pregnant she moved back to KS, immediately. For the majority of our relationship they never seemed to get along. She would blame him for not being involved when she lived states away, but ignored all of his attempts to be a part of his sons life at a distance. The only way he could see his child was if he flew to NY and stayed with them to visit. Which he did, multiple times. Fast forward to recent times, I found out that on many of these trips they were engaging in intercourse. I also found loads of inappropriate text messages between them. There was definitely emotional cheating going on, specially when she was allowing him to see/call the child. If he didn't make her his priority, then it would stop and they woould go back to arguing and she would withhold his son.

Now that she lives here again, she still only allows him to see their son if she is present. She makes him come to her home, but only on her terms. She claims because she doesn't trust him, but he's a loving father and I believe it's because she uses her child as an excuse so that she can remain close to him. 

she has mentioned to me that "she can have him if she wants him". She has tried to destroy my relationship with him by convincing him that I am manipulative and toxic. She is unapologetic for her role in the affair, as she believes she has rights to him since she has his first child. She also will not allow her child to know about our child and wants nothing to do with us. She is determined to keep us an unblended family. 

I have serious insecurities about their relationship and I am trying to suck it up for the sake of my relationship. But I don't trust either of them. They have both disrespected me and I am struggling with what boundaries would be appropriate to set. I want to give a fair chance to him because he says he has changed the dynamic of the relationship, but I feel like I need "proof" of that. She wants nothing to do with me, I have tried to establish a relationship multiple times. He doesn't want to discuss the relationship with me because it upsets me. He thinks that I should be okay with them being "friends" and having a friendly texting/phone call relationship. I do not agree with this. He has broken my trust and appropriate boundaries to me would be cutting off all communication unless it involves their son and him not having to be around her in her home in order to see their child. (She has court ordered custody so we are at her mercy).

To be honest, I think she wants to be with him and uses their child as a ploy and he doesn't see anything long term with her, but doesn't want her to be with anyone else because he fears that she will take his child out of his life completely. But I also believe there are some feelings involved on his end too based on some of the text exchanges I saw  

I would really much rather just leave this relationship and not deal with any of this crap, but I have a child with this man and we have a great relationship outside of this that I feel is worth fighting for first. I refuse to marry him until this is resolved. 
 

I'm looking for reasonable advice on appropriate boundaries to set and how to discuss this with him in a loving way without attacking or going down the road of rehashing past issues. 
 

please help.