If you can go back to your tester and also ask what grade level his scores would place him at the 90-95th percentile, that will give you a better idea of what appropriate placement would be. For example, if he were to place at the 90-95th percentile for students at the end of 2nd grade, then 3rd grade math would be a good place to start.
As for handwriting, aside from the "showing what you know" thing, from a practical standpoint, he'll need to write things down when he can no longer hold everything in mind. For many kids this happens as numbers get larger in 4th grade or so. It happens again when problems get more complicated in algebra and again when writing geometry proofs (which is more showing your thought process than not being able to hold everything in mind).
