at that age I would recommend, BJJ, Wrestling or Judo... in that order. Then as soon as he is old enough to join the wrestling team at school DO IT!
Nothing like free training... or training you are already paying taxes for.
Boxing and/or muay thai when you feel he is old enough to get punched... which might be now.
no offence... but if you have any white trash genes in your family or your wife's.. please dissregard everything I just said and have him do some non-combat sport, or a very traditional Judo school.... Otherwise you will create a bully.
I am J/K!
hard work usually creates very admirable characteristics in kids.