Boyfriend texting his ex? (Long)

I need opinions. I asked my fiancé if I can see the messages between him and his ex bc that is his ex & he’s always been a tad shady about things w her. He immediately got all weird & defensive asking why I was arguing or jumping to conclusions & asking why I was this way. When literally all I said was “can I see the texts?”

Anyway he let me eventually but while I was reading them he just locked his phone and said “that’s enough” like wtf? I asked him why I couldn’t read past that certain part and he just replied “because I said so” 😕 I told him that’s just even more suspicious like he’s hiding something he doesn’t want me reading but he said I should just trust him. How am I supposed to trust him when he told me he never text her& now he’s hiding conversations? Literally any time I get a random message from my ex (who I have a son with) I show him the whole conversation . Just out of respect

I told him I didn’t want to be with him anymore & asked him to leave. But he refused saying I was overreacting and that he wasn’t leaving when his kids lived here.

We have a daughter together and I’m pregnant , I also have a son I had before our relationship . I guess another thing that really bothered me was in the conversation she was saying rude things like “fuck them kids” or things about me. and I don’t know why he still bothers speaking to someone that says those things about us. I’m really on my last straw here and need opinions. Am I overreacting?