Good luck with teaching him some self-defense. As others have already said, the usefulness of training will depend on the school. I trained in Tae Kwon Do at a dojang that was great for kids. It not only built confidence and skills (I had a six year old girl almost choke me out, but grant it I wasn't fighting back), but it also stressed teamwork and honesty and other things you probably would like your kid to learn. At my dojang, they even got monthly updates from teachers at the students' schools and copies of report cards. If they got into trouble, they weren't allowed to come to class until they made amends. So it could also be a good motivator for kids to behave and do well, too, but only if you're at the right school. Good schools will let you try out a class or two before you join and you can usually get a good sense of the place after your kid goes through the class and you have time to talk to the other parents about how they like the school.