MY ex just texted me hey how have you been and it’s not even been a week that we broke up. What should I say should I keep it basic and not make it seem like I missed him a lot
I think you can text him back with a response and ask how he's been. Keeping things casual for now might be best until he decides to bring up the relationship. Then the two of you can go from there and hopefully work things out.
From what you said in the chatroom, that he hasn't sent a text back after asking for some space, then it's okay to believe that he's taking his time to think things through. Wait at least a week or two before asking him if he'd like to talk again.