I'm not an expert but to the best of my knowledge...
He would need to be 6 to take the WISC and if you suspect he's PG I would imagine he's be hitting ceilings in the WPPSI. The extended norms aren't out now but as long as the tester keeps going as long as your DS can and you have the raw scores they can convert them when the extended scores are released.
The norms on the SB are very old and finding a tester that still uses it might be a bit of a challenge. Admittedly our schools won't accept the results here anyway so that might be part of the reason why testers don't use it here. Perhaps in other areas that isn't an issue.
I don't know enough about the other tests to comment on them. Both of my kids have done the WISC and WIAT and of the two I'd pick the WISC for usefulness in both of their cases. TBH though I don't know that either would really help me if I was homeschooling other than giving a firmer reason why they were able to blow through curriculum that was *supposed* to take ___ days/months/years. At this point it already sounds like you've come to grips with that reality
Access to programs - definitely. That was the main reason we tested (that and it woke us up to the fact that our kids were maybe a bit more than a little smart).
FWIW I will also say that we tested DS at age 6.5 and because he wasn't the most cooperative at the time we ended up having to redo testing a couple of years later. The second set of scores make a lot more sense given our experience with him. We didn't think DD would be as uncooperative but we still waited until we needed scores for an application at age 7 in the hopes of getting an accurate score and avoiding retesting.
Good luck!
