Truth is, he can't have a relationship as long as he choses to drink over those he loves. Sure I have no doubt they love but that love is pushed so far down by all their own shame and self loathing, it cannot possibly be shown until they work through their own issues. You can't express love to anyone if you don't express it to yourself.
I don't think it is so much a choice as it is an action of default. A very ineffective way to deal with life's struggles and any emotional demands placed on them by family, work, friends etc. It is a cowards way of saying "I can't deal with anything right now but rather than think about why, I am going to numb myself".
Hang in there.