I agree, the "maxed out of the curriculum in the building" sounds like an excuse to me. I mean, they must have advanced 3rd graders. What do they do with them? Is there a GT teacher or a reading/math enrichment person? They should be able to develop a curriculum for him.

However, it is my understanding that public school is often not a good match for PG kids, perhaps in part because of the asynchrony you are describing. So maybe research some alternatives, if there are any?