My approach to this has always been to say that philosophically we use a "mastery-based approach to education." In other words, we don't consider the kids to have moved ahead in the 'curriculum' until they have reached full mastery at the current level. That's not to say that they don't jump about into more and less advanced things based on whims and interests, but I wouldn't report on that more advanced learning until there's some fairly cohesive mastery up to that level. So when middle dd was dabbling in a 9th grade math program at age 11, but clearly wasn't totally mastering it, I reported her as an A student in 7th grade math since that's where I considered her "full mastery level" to be (and she was nominally only a 6th grader at the time).
Three of my kids are in high school for at least some of their course-work this year. Their report cards just arrived today, and they're totally in line with the straight A's I was awarding them in past years, so I feel kind of vindicated. Middle dd, now 13, is acing advanced 9th grade math for real this year, with the top mark in the class.
Anyway, it sounds like your kiddo is similar. I think you should have no qualms about awarding him A's for above grade level work done well.
"Mastery-based approach" ...that's such a great description of how we're doing things. Thanks Miranda, that's wonderful!
And congrats to your kids on their successful transition to school!