Have you considered trying a different achievement test? The Kauffman or the Weshler ones? It might have just been an off day for him, or he didn't click with the tester.
I personally can't recommend you put a kid like him into public school. There's a slim, slim possibility it will work out, and a rather large possibility that it will be crisis-inducing for him. I can't see that it's worth the risk. Have you looked into Boy Scouts? Or martial arts? Some other sort of group activity that he's not previously done, and that's isn't directly tied to intelligence. Something he can connect to other kids his own age with.
I don't know how close you are to a decent-sized city, but you could check your local chapter of Mensa and see how many kids they have involved in their program. Also, you could check if there are any robotics teams in your area. They are often available to kids and adults of nearly any age.. so he could be working with teenagers, college kids and adults.
