Would you be ok with your boyfriend going to an event with his ex and the child that they share?

So my boyfriend told me that his ex asked him if he would go to a monster truck event with their son. All 3 of them going together. I like the idea of his son getting to spend time with both of his parents together but the more I think about it idk if I made the right decision. I’m 5 months pregnant & my bf has not told his son so obviously the ex doesn’t know either. I ended up changing my mind & telling him that I would only feel comfortable with him going as long as he tells his son that he’s going to have a little sister. Idk why I said this. I just felt so uncomfortable that my boyfriend hasn’t told anyone but his parents about the pregnancy. Idk if I’m thinking clearly either. Should I just let him go either way? He’s never given me a reason to not trust him but you know all those stories we read here. Also me going with them is a bit out of the question as I am a couple states away at the moment.