How should an 18 year old boy be punished after putting cyber friends before his family?

My 18 year old brother recently started to get friendly with a bunch of people over the internet which led to them arranging a meet up in Bristol, UK (we live in London). Despite my mum, who suffers from high blood pressure, constantly telling him he shouldn't and couldn't go, he left anyway saying that he will always put these cyber friends before his family (He travelled alone on a 3 hour train journey to a rough area of which he has no knowledge of). From what has been read over chat forums and the like, it appears one of the girls that is going has promised him sex and a pub crawl that will proceed over the course of 4 nights. There are around 30 different people that have gone on this trip, some of whom do not live in the UK and others who have made it clear that they dislike my brother for being arrogant and childish. My Brother is very immature and violent at the best of times but it is far far worse after he has had a drink (and he is only small so it doesn't take him long to get drunk). I understand that he is 18 and wants to assert his legal right to drink and have sex etc, but he has gone against everything his family and extended family have advised him and everyone has decided he needs to be punished, not to mention he has put both my mum's and my 78 year old nan's health in a critical situation due to the overwhelming stress he has caused them.
How would you discipline him if at all?