Thanks LexieCat -- you are right, it would seem cold to just stop taking his calls, so I will wait and see how it goes, and if I do decide I want no contact, I will tell him so.
The question has become moot in the last few days; I haven't heard from him or about him. While I do not know for sure, in all likelihood, this is because things are not going well with him.
I pray for him. I now understand a lot better that there is nothing I can say or do that will mean the difference between his staying on program and in recovery or relapsing. I am much better detached than I was previously.