[stuttgart46]

IIRC he will be in a Cadet. It's a lot of fun. Based on my experience, I would suggest that you
A. Practice a lot before you throw him in to a race with other kids. Some of those kids he will be racing against have been doing this a while and they are fast. It can be very intimidating and discouraging to get lapped by the pack your first time out.
B. Talk to Stuart Robinson at Kart Nation at MSR Houston. This guy knows his stuff and he will be able to give you a realistic approach to the sport. He can also help you move forward to cars as your son gets older.
Best of luck and please keep us posted.
My son did his first Spec Miata test on Saturday and it was great. One of the most emotional moments I've ever had at a race track.