Re-testing on a short interval is generally not recommended, as it places the results in doubt, due to re-test score inflation.
There is no obvious reason that a test on an 8-year-old would not be valid. It is true that scores in a young child tend to be less stable. If you plan to test exactly once, then, yes, probably wait until he is 9 or 10, when scores will be mostly stable, but there will still be a good ceiling for an HG+ child on the major individually-administered instruments.
I'm sure you could find many examples of not "evening out" in a quick scan of this board!
For recognition by your district, the simplest would be to have a district psychologist do it. Also has the advantage of being free. If you don't feel comfortable with the PS testing, a university-associated clinic is probably the next-most-accepted source. I seem to recall that you're in the PNW, in which case you could try the Robinson Center at UW-Seattle.
