Your son may not be ready to work as a mathematician next year but he certainly seems very talented and maybe his super working memory has something to do with that.
ETA: As for another IQ test, my own personal belief is no testing unless it's educationally necessary but that's just me.
You're probably right that there's probably no real need to ever have another IQ test. I do wonder, though, if that's his real profile of scores, or if there were some testing glitches, or if he'll naturally grow into being stronger in the currently average areas. I also wonder if this score profile is more typically of someone mathematically, rather than verbally, minded.
