I strongly recommend Tae Kwon Do (or other martial art) for children of all personalities. Those who are timid can learn how to speak up and stand up for themselves, those who are aggressive can learn to channel that into more effective behaviors. Good instructors will stress that these techniques are for in class and for self-defense and that's all. My school's instructors will tell the kids that they will be dismissed from the karate school if they use this on their friends, siblings, or schoolmates.
I have trained in Tae Kwon Do for a total of eight years. I am currently a Black Stripe working towards a Black Belt, which I hope to earn when I'm 50! My daughter started at age 5 and now at 8 is truly enjoying the physical and mental challenge. I taught kid's classes for two years previously and saw all types of kids benefit in different ways. Teaching Tae Kwon Do to children was one of the most rewarding experiences I've had in my life.
Before you commit (or pay) for a school, I would sit in and watch classes, speak with the instructors, and have them meet your son. They should be willing to give him some type of "introductory" lesson, even if for 10 minutes. Some might give you a two week trial period, for example. You should feel comfortable with the approach the school uses. Check the class schedule to be sure it will fit with your current schedule. Ask what the expectations are for attendence (once/week, twice, or more).
Tae Kwon Do is my therapy! I have been recommended to be on meds for ADD, but a karate class session focuses me just as well, and with only positive side effects!