Advice/someone to listen to an emotional mommy-to-be

So since I have known my boyfriend he has always taken care of his son and the mom is in the picture whenever she decides, which is usually for family gatherings to show off her son. Since him and I got together I told him it's not right she can't just decide day-of she's taking him and stuff like that. She always got her way cuz my bf never wanted to fight with her or her try to take custody. Now that she hasn't been getting her way, which also included her wanting them to go out like a happy family together and didn't want me around her son, she's talking about keeping him. We went to a parent teacher conference and all the pictures he drew he said were of his dad, me and himself. This pissed her off even more. Which I completely understand, I would hate to see my son getting attached to another women. I'm not trying to replace her but it's not hard; I help him with his homework, feed him, and tuck him in bed every night and on mornings I don't work I wake up to take him to school. Even when she does pick him up from school or ask to have him she tells my bf she's dropping him back off the same evening because she has plans to go out. She's been threatening to keep him and I told my bf we need to do something and he doesn't want to fight with her. Now she called him saying she's keeping him and going to make him pay child support. She knows I'm pregnant and she just told him she is too and why did he move on and all this shit.
I'm sorry for the long post I just have no one else to talk to about it and my emotions are out of control. I just wanted someone to listen, give advice or just tell me everything is going to be ok..