What I am learning is that the boundaries have to be around ME, not them...(sometimes, a moving, gray line).
In this case, I might say - I need you to be sober. I am willing to be apart from you until I see you are sober for one year. After that, we can consider reconcilliation.
In this way, you are not telling him HOW to get sober, only that YOU need sober people in your life. No one knows what works for different people... rehab is an option, but not the only one that works.
I wish you the best.