Regarding leaving things open-ended - yes! YOU can choose when to have that conversation, if at all. Reading between the lines, it seems that he wants to have this discussion to rearrange some of the existing verbal boundaries to suit his current situation. That's probably why he's pushing it so hard and fast. Are you happy with the way things are right now? If yes, then you have the upper hand in any negotiations. You don't have to be forced into anything. There's no hurry. Take 24 hours at minimum before responding to non-urgent communication. Write a draft and sit on it. Post it here and delete about 1/2 of it because I guarantee you don't need it all. "Reply in haste, repent at leisure."
One thing that really helped me distance myself was to stop referring to 'us' in any way, shape or form. There is no 'us.' There's you and D4, and him and D4. There's no team, you aren't working together in D4's best interest. He demonstrated that when he flouted your verbal agreement. In my case, I went out of my way to reword anything in writing that phrased it that way. eg. "We have shared parental responsibility" became "I have shared parental responsibility with you". It seems like such a silly, small thing, but it took practice and commitment to make it work, and eventually it made a big difference to my mindset.
In your case, you could write "I want to be on the page with you..." By saying "I want us to be on the same page..." you are still communicating how you want HIM to think, act or feel. Makes sense?
I added the bit about him being welcome to share his thoughts via email because it makes it clear you aren't controlling his actions in any way or denying him the opportunity to talk about co-parenting. Your response might come off a little dismissive. Remember, he can do whatever he likes. I agree with Own - it's a good thing he is putting in effort with regards to parenting. You are simply declining to participate on his timeline. It's not never having the conversation, it's on your terms. He sends an email, you read and review at your leisure. It removes the emotions and the urge to engage. It also sets up a precedent for communicating about D4 in writing to avoid confusion over verbal agreements in future.
