Looks like your last post crossed with mine/was hung up in moderation, but just in case...
Your DS must be six to take the WISC at all, as that is the beginning of the age norms. Extended norms for the WISC are expected out later this year, which should address the ceiling issue for the WISC.
The WPPSI does not have extended norms, and may have ceiling issues for GT kiddos. To minimize those effects, in general you would want to test sooner, rather than later. (Its age range is 2-6 to 7-7, with two brackets: 2-6 to 3-11, and 4-0 to 7-7.) Since your DC is just under a year into the second WPPSI age range, the ceiling effects may not be as severe as expected. (He's in the beginning half of the age range.)
