One of the hardest things to accept is a bad return on an investment. You know that he has the power to give you a good return on your investment, yet is choosing otherwise. It really is like being used.
At the same time, for him to find sobriety and not relapse, he has to limit his views of himself to the positive of what he can become. For awhile at least, I think that in early recovery they have to release themselves of any guilt, or they may relapse. I'm talking about guilt coming from within. Any guilt that you may convey in your letter is not why he would relapse if he does. That you feel you got a bad deal out of it all, he knows already, and wouldn't be surprised at you taking jabs at him.
Thing is, it's out of your control. Write the letter as if he weren't even going to read it, and that he would throw it away. Realize that is an option for him, he can read it and take it in, or he can reject it. It's his power alone to decide that.
What would you tell a person that invested in the real estate market 8 years ago and is now penniless with the fall? You'd say--reinvent yourself.