I would not be sending any letter to him.
4 months is very early in his recovery, so I don't imagine all the addict personality is out of his system, but what I see, and call it what you want, he is certainly trying to engage YOU. Some call it baiting, I call it set the hook, and reel you in.
The actual percentage of people who recover is very low, I would not be holding my breath. I would not be juggling any of the current fire balls he has tossed your way.
You say he is intelligent, well then he certainly can figure out what moving on means.
So my question is are you serious about moving on? Because if you are, no further words are needed.
Be careful.