The law in the UK does allow the use of reasonable/necassary force to defend oneself from unlawful force. I believe that this is much the same in the US.
However it is deemed illegal if you duel, which being called out effectively is. It could be regarded as an unlicenced boxing match, which is also illegal.
If you are so bothered about it, why don't you ask him into the ring? Mind you I doubt it would do anything for your reputation, kicking a 16yr old's ass.
Don't worry too much about it. If the harrassment continues inform the police. They're paid to deal with idiots, you aren't.