I’ve asked for single subject advancement in elementary but they said one grade level bump doesn’t even do him any service because he’s several years advanced and there’s no prebuilt path to do that. (For example, you can’t take 6th grade math in 4th grade b/c 6th grade math is taught in another building. Also a super advanced 4th grade reader doesn't necessarily want to read what’s socially/emotionally appropriate for high schoolers.)
He will get math acceleration in middle school, but there’s a track to do that that’s prebuilt. with that, he’ll end up in college math classes mid high school. Skills wise that seems fine, but there’s a lot to manage logistically - different expectations, different location or online, managing 2 school systems simultaneously, exam timing, etc. That’s a lot to ask of a 16 yo that may or not be able to drive yet.
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The internet provides access to a huge range of maths content. There are lots of interesting materials in the Numberphile videos, whilst Khan academy is curriculum based. Here in Australia, the AMT, which administers the AMO to select the national team for the IMO, provides resources aimed at mathematically able students from third grade upwards. I searched MAA, the counterpart organisation in the US, which administers the USAJMO and USAMO, to look for resources for your DC, but it seems that one has to join to access the full resources (
https://maa.org/audience/students-k-12/). Might be worthwhile for you and your DC to look into this.
