I think you want to do it kindly. Maybe just say something along the lines of needing some more quiet time to get your own self together, and it's just too hard to do when you are so involved in caring about how he is doing. I think you want to let him know that you DO have the very best hopes for him, but that you need time strictly for yourself right now, and that you hope he can understand if you prefer not to be in contact with him for awhile.
And then USE that time, exactly as you said, to go to some Al-Anon meetings and focus on yourself.
If you simply refuse to take his calls, with no explanation, it seems kinda cold, you know?