boyfriend still talks to his ex

I’ve been with my boyfriend for 6 months now. He talks about how gross his ex was and so much trash about her. But I just saw text from them that go back to when we first started dating. He told me he doesn’t talk to her. He said sorry to her about being a dick to her and they talk about their past relationship. They even talk about meeting up for a class they’re taking. He did not tell me about this. What do I say to him?