As stated above, I would look at what the last two years have been like and decide if you are willing to risk getting pulled back in again.
Also, seeing that he has still not dedicated himself to sobriety, it seems like the rollercoaster will continue. Although you have decided to only be his friend, our emotions don't notice that distinction sometimes. He might be a good friend sober, but what happens when the drinking increases and the black outs become more often then not? Are you willing to have to deal with that again? Is the friendship worth the constant worrying and having to block calls and feel out the situation to see if it will be a night where he is drinking or not? Lastly, I just want to point out what a selfish disease this is, he might say he understands what your boudaries are, but as his drinking progesses, so will his attempts to get you to feel bad for him & to comfort him. As he goes downhill he will try to pull you in and say things like "I need to know i'm loved/cared for/etc." in order to get attention/help/bring out the enabler in us all.
As always, whatever you choose to do we will all be behind you
Just remember to put yourself first and as soon as you feel it is getting to be too much, please do yourself the favor and end the friendship. We must always protect ourselves.
hugs
Maylie