I've messed up. What do I do?

While I think that it's really wrong that you let him smoke pot, I don't think that's the only cause of his behaviour. I mean, there must be something else behind that, pot doesn't transform people into complete others. You should talk to him seriously and tell him that he's going overboard by doing all this. Maybe he's trying to be a rebel or something like that, it hits a lot of people during their teenage years, but it never lasts that long.