I have to say having him take a martial arts class is an excellent idea. Not so that he can go around and beat kids up. He will learn self defense of course, but also self discipline and more important self confidence. Martial arts don't teach kids to beat people up. Maybe they have a short self defense class the two of you could take together.
My daughter took kung-fu for a couple of years and it did wonders for her self confidence, best thing she ever did. She was at school one day and a kid was picking on her and her friends, she repeatedly told the kid to stop and the kid didn't. They both ride the same bus home and after they got off the bus she through him on the ground and started pummeling him in the chest. She never had any problems until one day he was picking on a special needs kid on the bus, calling him names, writing on his backpack. She told him that was wrong and he better stop. He didn't and after they got off the buss he got a beat down. He asked her if she could at least wait until the bus left because all his friends were making fun of him. She won't take crap off someone, but neither will she start something.