I think my DS spoke math before he spoke English, he just loved everything about it.
Every teacher is different of course, but our experience has been that nothing destroys a child's love of math faster and more effectively that years spent in elementary school doing repetitive drills of rote computation. So my very personal two cents worth: if there is anything you can do to shorten the amount of time spent on those very basic skills and move more quickly to more substantive math, take it.
With most of the year to prepare him, I would be surprised if you need to spend a lot of time prepping him specifically for the content of the end-of-year test. But that would be my one caveat - if test prep starts showing signs of ruining his natural love of math, then back off. But if you can sneak it in with the fun stuff you two are doing anyways, do so. There are so many fun, wonderful resources out there where he can learn math in interesting ways, and which you can then supplement with the minimum necessary "how to format this on the test" stuff.
P.S. Perfect? Seriously? Rigged indeed. But worth trying.
