I'm not convinced the problem is 'obvious' at all. Four times floppy doesn't constitute impotence, especially when those four occasions all come close together and have been preceded by months of normal sexual activity signed off with a happy orgasm.
So what might be going on? I'm guessing your boyfriend has simply got his knickers in a twist over that first drunken failure to maintain his erection. After 'messing up', I'm betting he became so anxious to prove himself as a competent lover that just when he needed to be at his most relaxed, his most untroubled, his nerves kicked in, sabotaging the moment. This is the sort of thing that happens when too much focus is given to sexual performance - maybe it has even happened to you once or twice in the past: you're so worried about whether or not you're heading for an orgasm, your pleasure never gets the chance to get off the ground. The solution for you and your boyfriend is simple: ease off. Don't put your boyfriend under pressure to reverse this dreary little trend that has emerged, and don't put him under pressure to talk about it either (after all, what's to be said?) Just make it clear to him that you support him and let him know that whatever you accomplish together, you find him incredibly sexy and exciting. In other words, don't let him feel like he's disappointing you. Try to steer the focus away from sex by spending your moments of intimacy cuddling and fondling rather than having penetrative sex (you can always use your period as an excuse, or say you're recovering from a spot of thrush, to take the heat off the moment).
This sort of blip in a bloke's performance history is usually short lived, so please stop viewing it as a permanent or irreversible problem - or even as a problem at all. If, however, the situation doesn't improve over the next few weeks (and truly, I see this is as an unlikely outcome), your boyfriend get some reassurance from contacting the Sexual Dysfunction Association (www.sda.uk.net) and arranging to speak to an doctor about erectile dysfunction.