Not sure if he is your first/only child but I have to say that I honestly wouldn’t worry too much.
I would think pretty much every teenager in the UK goes out and does this from the age of about 16 onwards? I know me and my brother and um, all my friends used to. I genuinely think I would be more worried if I had a teenage son who didn’t do this at least a few times.
There’s no way he’s going to see it as a ‘problem’ as it probably isn’t- he was most likely having a great time until he got smacked in the mouth. And after a day of having a hangover and a sore lip he will surely be fine. Lets face it, he’s only doing what half the population do every single weekend. unless you think he’s actually an alcoholic or something I wouldn’t worry.