Does he have the support of other addicts/alcoholics in recovery? Is he working a program? Or is he just bulling through/white-knuckling on his own willpower?
I get the sense he is doing it on his own. If he relapses, I'd wait till he's sober-ish and tell him that you care about him, and that you hate to see him doing this to himself. Suggest it may be time to do something different. You could offer to call someone from AA or NA to come talk to him, and offer to stay for the talk if it would help make him more comfortable about it.
Beyond that, I don't see that there's much you can do, other than look out for your own well-being so you don't get wrapped up in the insanity of addiction.