I was in the exact same place you are when we (well, I -- ex-h wasn't on board in the slightest) decided to test. I knew he was smart, but I couldn't decide *how* smart, and what I was justified in asking for from the school. Talk about opening a can of worms!
My ds6 took the WPPSI at 5y5m and ceilinged 7 of the 11 subtests, leaving him *just* short of the DYS WPPSI cut-off. If you're hoping (even a little) for DYS scores, I'd wait for the WISC, or just do the SB5 -- from what I've read of your ds, he's almost certainly at the higher end of the spectrum and the WPPSI just might not provide enough ceiling room. It's possible to be accepted to DYS with close scores and a portfolio, but it's kind of a pain in the neck.
When you have a child who you are pretty sure qualifies but misses the cut-off on what amounts to a testing technicality, it's frustrating knowing that the resources there are just out of reach.
I've said this before and I don't have any specific research to back it up, but two GT-experienced testers I've contacted separately recommended waiting on the WISC until closer to 6.5 or 7. Not sure the rationale on that, but I thought I'd throw it out there.
