Welcome, M&M!
Obviously, those are strong test scores. The one correction I would make is that his FSIQ is unlikely to be correct, as that would be substantially higher than suggested by his index scores. A glance at my manual indicates that his factor scores would result in an FSIQ of 134, which corresponds to a sum of scaled scores of 152, which is probably where you saw that number.
My second comment is that the age-equivalents are not the best measure to go by, and are actually specifically discouraged by the APA and NASP (the two professional organizations which accredit assessors of cognition). The discussion below is based on percentiles and standard deviations, not age-equivalents.
That being said, he is still clearly a highly capable child, with a particular strength in the visual-spatial area (this lines up with his artistic giftedness very well). His fluid reasoning is comparable to his visual-spatial gifts, though insignificantly lower. The remaining three areas are around a standard deviation lower, falling just above and below the beginning of the moderately gifted range. I would say he is most accurately described as highly gifted in visual-spatial areas (and possibly fluid reasoning), and moderately gifted in the other areas.
This suggests that the academic profile you see at the moment does reflect his cognitive profile, with the exception of careless errors affecting his on-demand mathematical performance. I would not be surprised if the careless errors were simply developmental in nature, or possibly from lack of cognitive engagement with the tasks, due to insufficient challenge. What you report of his reading ability is consistent with where his knowledge index falls, as the best available estimate of reading-related ability (since we don't know his verbal and nonverbal indices, it's not clear if there is a broader verbal/nonverbal profile). The data do not suggest that he is underperforming in reading, though he may be in math.
Has he been more thoroughly evaluated in terms of academic achievement? It can be helpful in advocacy to have objective data on academic skills.
