It's not so much as the style as the instruction and the stdent. If you have goals for your kid, then you, as was said before, should shop around for the best instruction to fit these needs. This can be said for any other activity for your kid whether it is martial arts or horticulture for kids.
You said you want him to gain focus and control, but these things don't just come with the territory-- your boy has to want it. Honestly, he can learn these things just playing a musical instrument, or playing with his legos.
If you're going to learn martial arts for the right reasons and you actively pursue an education with the assistance of a good instructor, then you're in the right boat, but if you're just going to throw yourself into the mix with hopes to get something specific done, then you can probably just forget about it right now.
When I was about 6 or 7, I was in a martial arts class, probably enrolled for the same reasons you are planning on enrolling your kid. To put it bluntly, I hated it. I basically learned nothing from doing it, and with good reason-- but when I grew older, I became interested again with specific goals for myself. It's been years since then and I've come away with a lot more then I could ever dream of.
But don't think I am trying to get you to stay away from martial arts entirely. I think it will be a great thing for your child to get active and use "his moves" in a structured manner. But you're going to need a teacher that understands why he is there-- as opposed to one who doesn't, who teaches your kid to use the moves he learns irresponsibly.
If you've got anymore questions or comments please ask them.