My dh told me he wanted to leave me when I was 7 months pregnant with dc2, three days before Xmas.
It was an appalling shock and the timing was fucking awful!
He did stay living in our home for nine months, finally leaving in September, because I desperately hoped that having a newborn daughter would change his mind. Sadly, it didn't.
I never went NC with him. I just couldn't. The dc were so young that I decided, no matter how or when he had done what he had done, I was not willing to take the risk that not talking to him would sour our inevitably long term relationship.
I absolutely know that going NC with him, even for just a short time in the early days, would have meant that we would not have the relationship we have today. My friends comment about how 'amicable' we are, considering, and how, despite everything, we are actually friends.
I could not, and would not, ever put my children in a position where they thought we hated each other. I am not bitter. I moved on pretty swiftly (as it seems the OP is doing). It made me a bloody strong person actually.
If you are feeling strong and happy, the fact that you have 'outpoured' your feelings to him, will probably mean that it won't 'churn you up' if he doesn't respond in the way you would like.
I totally agree with FantasticButtocks last post.
A quote from one of your posts today - 'It just seems so formal and unnecessary' - it really doesn't have to be this way you know. It doesn't make you weak, emailing him you know 
Seriously, go with your gut. Do what you think is right for you, your son, and to a certain extent, your ex.
I wish you luck
x