I don't have a lot to offer in terms of your question, but it did bring to mind two thoughts:
1) Is he taking the WIAT (Weschler Individual Achievement Test) or the WISC (Weschler Intelligence Scale for Children)? The former is an achievement test and the later an IQ or intelligence test. The WIAT would have grade equivalencies, but the WISC would not.
2) I've never heard of a tester telling the kid who is testing how well he did in terms of 'you made it to 7th grade level,' etc. during the test. I'd, honestly, be a little bothered by this as it might diminish performance in a child who is perfectionistic.
I can certainly see as how achievement level on English reading/writing tests might be lowered in a child who has been in a second language immersion school, FWIW. In terms of ability/IQ scores, I don't expect that immersion in a second language would lower scores on most portions of the verbal comprehension index of the WISC except maybe vocabulary. It may be a plus or a hindrance on that piece.
