I would second polarbear's recommendation. Instead of looking purely at ADHD, you would probably be better off looking at a comprehensive evaluation, as children tend to display the same symptoms for a wide range of challenges (due to their smaller repertoire of coping skills). If you want to know about working and other memory, there are other scales that will go down to age five (i.e., you don't need to do the WISC to get working memory info), many of which will give you much more in-depth info on his memory profile. I like the WRAML-2 (Wide Range Assessment of Memory and Learning) and the CMS (Children's Memory Scale). Another advantage of this approach (WPPPSI and WRAML/CMS) is that, in the event that ADHD really is involved, and he gets put on meds, you will still have the option of doing a re-test with medication more or less immediately, using the WISC-IV or -V, rather than having to wait for two years.
FYI, there is no formal lower limit on ADHD diagnosis, though many practitioners are reluctant to diagnose and medicate in pre-elementary children, due to the wide range of normal in preschoolers. The actual age-restriction is that symptoms need to have been first reported in childhood, not appearing suddenly in adulthood. (It used to be before age seven; now it's before age 12.)
