Okay Jacket here I go giving (what is probably) terrible advice.
But ignore him. Just ignore him.
It sounds like he is going to write AND send this stupid email no matter what you want.
And, by the way, this email is stupid.
He is going to deliver a narrative about your marriage that no one wants to hear. It's awkward and embarrassing.
Married couples don't issue press releases because no one wants to read them.
No one.
Not you. Not them. No one.
And deep down he knows this. Which is why he wants you to buy into this idiotic idea. So when people question this email he won't take all the blame.
What a wimp.
Stop responding!
Then when people ask about the wording--of this remarkably embarrassing socially awkward email WHICH THEY DO NOT WANT TO RECEIVE--you can say (with conviction) he wrote it.
What is the story you want to tell about this a year from now?
"I helped my husband write an embarrassing, socially awkward email about our separation which people will gossip about for the next 3 decades."
Or
"When my husband decided to write an embarrassing socially awkward email I expressed my objection. But he was determined to go through with it. He is an adult. I do not control his actions. Nor do I want to control his actions. That is not my role. Nor do I want it to be my role."