Welcome Back to SR.
So what do I say when he calls begging to come home?
NO
No is a complete sentence. To help with that, go No Contact for a bit, just don't answer his calls. Maybe you might like to find some Al-Anon or Naranon meetings in your area, I suggest Al-anon as there are usually more of those in any give area than Naranon.
Unfortunately you cannot help him any more than you have, he has to do this for himself. You have the right to set your OWN boundaries. Al-anon or Naranon can help you with this.
There are also the
3 C's
You didn't
CAUSE it.
You can't
CONTROL it.
You can't
CURE it.
Writing a letter is not a bad idea, I would suggest though that you write, then leave it sit for a few days, go back and re-read it, edit, leave it sit again for a day or two, than do the same thing again. By then you will know within yourself if you still want to send the letter.
Hope the above helps a bit and doesn't 'muddy' the issues further.
Again, Welcome Back.
Love and hugs,