Hi T! Welcome back! I understand the absense. I have yet to lock a thread; I use it to work through some things and then just get tired of being mired in all of this yet again.
Anyway-
My opinion, for what it is worth, is to give the typed version of the letter back to your son and let him do what he wants with it. Take your stuff off of it and tell him that it is a copy of the words he spoke to you that night.
If you involve yourself in it it will seem like emotional blackmail and the message will not get through.
As for your letter--- Don't. It is great that you were able to articulate your feelings, and it is good for you to do so, but you need to now follow the advice of my good friends Betsey and Meredith and put that away in a box with all of the other letters you will most likely write to him. Sweetie, there are no magic words that will bring him home. Is there anything in that letter he doesn't already know?
I hope others will chime in. From my experience, and oh man was I a letter writer, they do nothing but cause more distance. And guilt.
I'm sorry you and your chidren are still struggling so much.
Hugs, Pam
P.S. I was just doing a report on young children of divorce, and you're right, it is said that younger children need to see the other parent at least every 3 days if a relationship will work out later on, as they do not develop the memory needed to bond with that person until age 5 or 6. Interesting, eh?