Unless you have a need to test now I would wait until he's a bit older. Testing at this age can be unreliable. You already know he's gifted. You don't need a test to tell you that. I would test before he's set to enter K (or Pre-K if you're contemplating early K entry) so you know exactly what you're dealing with and then present the results to the school so appropriate accomodations can be made. Without a GT program in place be prepared to fight for what your son needs. In the meantime just keep doing what you're doing. Follow his lead and keep providing him with new opportunities to learn. He sounds like an amazing little boy!