The implication would be that he maxed out the test, with skills of at least seventh grade-level. The manual suggests that a fourth grade test would not have information on skills above seventh grade. Keep in mind that the grade-equivalent tells you what the median 11.7+ student would score on the same test, not that there were actual grade 11 & 12 items completed correctly.

Is there any reason to believe his scores would be an obstacle to compacted 7/8 math, at the moment?

...pronounced like the long vowel and first letter of the alphabet...