What should I do?

I live with my boyfriend, the father of my baby and due May 7th. Well his ex forced him to stay to sleep at her house for just today or when she want to. They have six kids but they're teenagers but not all are his blood kids. She told him that if he do no do it then will seek more child support to give her more money. Should I leave him or understand him? I'm not going to let a man to sleep w his ex and me at the same time. Like he sleep w his kids supposedly but I don't believe him because his ex might want sex and he might do it to calm her down. Or idk I'm just saying because men can lies. I don't know what to do. I never thought that I would be single mother someday. Please advice...