You should definitely tell. It's bad enough what you did, but now you're going to lie/hide it from him? Pretty FCUKED up. As for how much a drunken kiss could mean? if this guy was a stranger it probably didn't mean much, but what it does mean is that you didn't care enough about your bf in the moment to control yourself. And well if you can't control yourself you should not be in a relationship PERIOD, or you should not drink PERIOD until you learn to have some self control.
"Sometimes the best way to throw a punch is to take a step back"~Morgan freeman