I told him to choose me or his kids mom

I told my boyfriend it was either me or his kids mom. His kids are 11, 12, and 15. They all have cell phones. She calls him everyday asking him to get back with her. She will send me messages saying he is trying to get back with her. He deletes all the messages from her. He never answer her calls when I am around instead he goes outside to call her back. I might be wrong but I don't think he needs to talk to her. He can talk to the kids they are old enough. I talk to my kids dad when I have an emergency with the kids or if they are sick. It's a so and so is sick took him to the doctor got meds.... he will respond thanks I'll text them. Same when he has them. She has no boundaries and I'm sick of it.

Update: just for clarification I was perfectly fine with communication between them. It was the hiding messages and only answering when I wasn't around. I wasn't even thinking anything of it or trying to read his messages until she told me that they were getting back together and until their 2 daughters told me they would lock themselves in their room because their mom and dad would be kissing. I'm not some jealous girlfriend that doesn't understand he was to be there for his kids. All I wanted was boundaries to be set and respected. I broke up with him last night. Told him he could stay until he found a place and he told me he would be moving in with his kids.... so yeah obviously with his ex. He was hiding something. I told him I wasn't going to argue or fight and that once our baby is born ill show him what actual co parenting is. Thank you for the advice ladies and thanks for letting me get it off my chest.