Did the tester mention extended norms? Your son certaintly qualifies for it.
http://www.gifteddevelopment.com/Extended_Norms.html

Subtest scores aren't technically interchangeable. You can use up to two substitutions when calculating FSIQ, but substitutions are only supposed to be done if a core subtest is somehow spoiled (test is interrupted, tester somehow screws it up, etc.)and therefore invalid.