Based on what I've seen of the WJ-III Achievement tests, around K-2 grade I'd expect that tutoring would make a significant difference, but so would ability to grasp concepts easily as well as an ability to work quickly. There are two or three different subtests that add into the Broad Scores under each category of math, reading and writing. Sooo... I wouldn't be so quick to assume it's *all* exposure.
Did you have a chance to see his subtest scores on the WISC, or just the averaged scores? An FSIQ of 100, for instance, could mean a child is right around average all across the board in all the different types of abilities tested, or it could alternatively mean that he's got some real strengths in some areas (could be up in line with his achievement scores or higher), but they are balanced out by weaker scores in other areas. It's possible he may be really strong in the abilities that go into math reasoning etc but not so strong in verbal, for instance.
Re the reading (and really any of the other subtests), I found it really helpful to look at the specifics of what the subtest asked to understand why my kids' scores fell where they did - it usually made sense.
Best wishes,
polarbear
