Technically, it's better to wait 24 months between administrations. That's the point at which the test-retest effect falls into a range comparable to normal standard error.
There are four subtests in the core that have analogs on the WISC-V (you need one more for the GAI, three more for the FSIQ, and six more for the five factor core indices). Typically, the nonverbal subtests have larger retest effects, so I wouldn't worry too much about two of those overlapping subtests, which are verbal.
I generally wouldn't advise retesting on such a short interval, but from the standpoint of test-retest effects, it's probably not terrible. I would be more concerned about whether your DC is more reliably testable now than he was only 9 months ago, because if you do the WISC now, then you definitely shouldn't be doing it again until he's at least eight, so if he isn't developmentally ready to give optimal performance, you'll just have pushed off a solid retest for another two years, vs, for instance, waiting until next summer to test.
