Welcome I2.
BURN THE LETTERS. EVERY LAST ONE OF THEM. I myself had two big boxes filled with two years worth of broken promises. He won't change until it is time for him to change. Choice or no choice, don't know, I have never been an addict so I can't speak for one. I can speak for me and tell you where I have been. My x has four dwi's, spent two years in prison and wrote the most beautiful, heartfelt letters. So full of promise, so full of potential. So full of $#/7.
When and if he gets sober is not up to you, it's between him and his HP. Nothing you can do or say will ever change him. I hope I don't sound too harsh but after the last two years of him being out I would not wish what I have experienced on anyone.