The WISC should be minimally impacted by his educational environment. What he has had is perfectly adequate to feed his potential as far as cognitive assessment goes.
The WIAT (achievement) hypothetically could be affected, but at this age, I would strongly doubt that there will be any noticeable disadvantage due to his lack of additional instructional advancement. Consider that multiplication fact fluency will not be assessed at all, and that being able to add and subtract double digit numbers will take him above the 98th %ile in math calculations. Geography, history, and science (the so-called content areas) will not be assessed at all on either instrument.
Keep in mind that he is being compared to typical kindergartners/rising first graders, who may have 150 sight words (optimistically), and be able to do very simple single-digit addition sums to 9.
