What you described is exactly how a colleague of mine acted the last few months he worked with us. Fine some days, disappearing for weeks at a time. He never did ask for help. Eventually he moved on and away. I don't know how his new job is working out, or his living situation. On Facebook, it seems okay, but who knows?
I guess what would concern me is that you seem to want some kind of relationship - but he's not available for one, and you've chosen to hang in there. You seem to think that you are the only source of comfort/sense/support he has, when in reality, he's done nothing to access help for himself, and it's readily available. He has a family, who are doubtless aware that he's disappearing for weeks on end, and aware of his work history, AA meetings are everywhere, and online if one lives in the boondocks. Other addicts may be able to offer something to him. A mental health professional may be able to walk him through whatever demons he's trying to drown in booze. Us mere mortals are probably most helpful by staying out of the way.