Well, I'd recommend starting with what questions do you need to have answered.
Beyond general curiosity and the (common) feeling that you're in over your head with this kid and grasping at straws, what do you *really* need to know to help him? What information are you missing that is making it hard to serve his needs?
Then you can consider what testing will best help you answer those questions.
My guess is that at this point, achievement testing is probably more useful to you than IQ testing. You already know the kid is "wicked smart" and is almost certainly beyond the useful range of any test instrument that's out there. Unless you're looking for LDs, I can't really see a reason that further IQ testing would be of much help to you. What you need to know is how far along he is already so that you can figure out what sort of educational help he needs.
But of course, YMMV.
