Does he need a certain score for access to gifted services? Do you have concerns about possible learning disabilities? Is he having some kind of trouble at school?

The block design could have been just a fluke... by itself, I wouldn't think it is cause for concern.

If you decide to have him retested, he won't be able to do the WPPSI again anyway since it only goes up to age 6.5 (I think.) You could have him retested with the WISC-IV or Stanford-Binet V.