R.J.
You know... I'm JUST starting the same for my adhd kiddo. Although we're doing Aikido.
What's NOT important is the STYLE of martial art, but instead the WAY in which it is taught. Think Karate Kid (the old one, haven't seen the new one). You know how there are the 2 dojo types? The guilt/shame/win at all costs/being hit in the head teaches you to duck dojo and the respect/thoughtfulness/best you can be/control -so you can stop a milimeter BEFORE you hit someone, or control the strength in which you hit someone dojo? Those are really the 2 trends in martial arts. Dojos tend to fall into one or the other. So the trick isn't the STYLE, but the STYLE IN WHICH THE KIDS ARE TAUGHT.
Something to look for as well:
In our area there is an Aikido school that focuses *primarilly* on Aspie kids. Yes, they take typical kids, adhd kids, etc... but the instructor has an aspie child herself, and so really gears the lessons in ways in which HFA / Aspie kids can get the most benefit out of it. It's really cool. So when you're looking for a dojo, not only pay attention to how the kids are taught, but also look for instructors which actively look for special needs kids.