Congrats on getting him registered for K! It was a really huge thing for me to grasp in my head... sending my first child off to K! The childhood years fly by so fast - enjoy them!!!
Re testing, jmo, but I'd wait unless you have a program you're trying to get him into that requires it. I'd wait simply because the reliability of the tests goes up slightly as your child gets older - having a set of WISC scores is going to be easier to advocate with than the WPPSI etc. Even if your child does well on the early testing and you're confident in the results, school staff may question them.
One thing you *can* do (and I wish I had done when my ds was very young) - is to keep a record/journal/whatever of the things your child does and says that are those lightbulb "wow, he's really gosh-darned smart!" type things - because you can use those later on when advocating for your child. Not just milestones like "started reading chapter books at 14 months".. but ideas etc that your child has that show he/she is thinking differently, advanced, creative etc.
Best wishes,
polarbear
