Quick story to serve as an example: One of the questions that a detox facility asked my son on admittance was, "Is this your first time here?" I thought, jeez, I hope there aren't any more. Well, there were. There were 6 more visits to detox so far. He hasn't consistently had insurance, so rehab hadn't followed detox until his last visit - he's been out of rehab for a month now and has fallen off the wagon 4 or 5 times, but was able to right himself after a couple days, albeit with close supervision by his girlfriend, my wife and I. I'd expect that if he's left alone for more than a couple hours, he'll be at the bar or getting a handle of bourbon delivered.

TL;DR version: It's great that your boyfriend is going to rehab, but don't be surprised if it doesn't resolve two decades of alcoholism. From what I've seen with my son, as much as he wants to stop because he knows that he can't function as a member of society while drinking, when he's sober, he just wants to drink.

Good luck to you both.