Welcome, eventemp!
If you already suspect that his relative weaknesses at this point in his development are speed and fine-motor, then he is probably better off taking the WPPSI, which has fewer fine-motor demands. The WISC does have a higher ceiling for a just-five-year-old than the WPPSI does, but the trade-off with fine-motor expectations may not be worthwhile.
They probably want to do their own testing for a couple of reasons, including equity (so everyone considered for the program is examined at the same stage and on the same instruments), and because development can be in fits and starts for little people. A lot can happen in one year for a preschooler.
