Brazilian Jiu-Jitsu, Dirty Boxing and common sense!
First, you obviously want to teach him fighting is wrong unless he is protecting himself. Personally, I like BJJ. But, it might be difficult to set up a throw or counter or apply a submission in a crowded area (bar,concert,playground yada yada). Especially when the fight could be broken up in less than 45 seconds. Boxing, Dirty Boxing is great!
Also, size can matter. Sometimes skill can not be enough if your opponent is bigger, crazier and has an unorthodox fighting style. Learning other ways to defend yourself on the street is mandatory, like ... breaking an antenna's off a vehicle and slicing... grab a tree branch and swinging.. biting
. etc.
I am not trying to be funny or sound crazy. I know this thread was posted for a young gentlemen. BUT you do not want him to have a false sense of security! Kids now a days are a lot meaner, have a don't care attitude and more street savvy then when I was young. Good day Sir.