Co parenting advice?

So Monday I found out my fiance has been cheating on me for 1½years with 10 women supposedly. I have come to the conclusion that I am not going to go back to him. Due to this plus all the lies, continued lies and on top of many others. This being said I need advice desperately as im trying to do right. Because his first action when I was telling him that the wedding was called off and I dont think I can see a future was to pack up and run to Montana. My dad stopped him and had us discuss this again because he didn't understand the whole Role-playing sexually then he was still gonna leave which I was fine with but his mom told him to try and make it work but ive already tossed his ass out with all of his belongings and some. Truthfully I want full custody but if he wishes im fine with him having that relationship with the kids and I told him he could and I won't seek for child support. He never responded when I told him while he was collecting his stuff. Worse case which im sure this is what it will be is both having to share custody with child support. What did you moms who went through this do? How did yall bring it up to discuss I've already started trying to do co parent with updates of the children after I finally decided that I was done which was Wednesday. I've taken my salty feelings out of the whole matter with him ive already been trying to let him see them and ive also started the night time video call. I'm just lost on what to do im trying to do right but im unsure of the steps. Plan to get lawyer but all that ive talked to in my town dont do it so I will be looking outside of my town. I just need help with how do I bring up the custody agreement. I'm scared he would try and take the children and run to Montana on the orders of his family. Any help or advice is much appreciated. Thanks in advance.