What should I do?

My boyfriend keeps talking to his ex. They have kids together. His ex lives at his moms house due to the fact she had no where to go. Him and I have been together 2 years. The other day he got a phone. When I would walk by he swipes out messages. Her and I have talked off and on. I don’t have anything against her. She messaged me the other day saying he asked her for money. Him and I have been back and forth about this whole thing. I told him I don’t feel comfortable knowing they talk like that. And not about the kids. I would have caught hell if I talked to my ex like that. Thing is he will stop talking to her unless its about the kids if he’s been caught. And goes back hiding doing it again and again. I just don’t feel to comfortable knowing they were talking like that. What do I do??