Welcome back!
Kudos on the thorough and even-keel manner in which you articulated your situation update... both the positives and the concerns. I have just a few thoughts at the moment.
If there is any disability, you may find great information at
wrightslaw, including and addressing classroom support via
IEP/
504.
Understood.org is another helpful site for explaining things in layman's terms, including
IEP/504.
he taught himself to read
While gifted kids may often be autodidactic (self-taught), they also benefit from having teachers and a supportive learning environment with peers.
DS's never-ending thirst for knowledge caused chaos at home because DD8 is aware that his natural skills surpass hers. Their strengths fortunately are different, so they often help one another, but his mathematical ability puts him at a level I am scared to find out. I've managed to quell the anxiety in my oldest, but I'm still struggling being mom to DS.
Very wise and insightful.

math work he could do on his own. I love DS teacher's advocacy.
Kids need both
appropriate academic challenge and
intellectual peers. The school planning only for self-taught math in isolation is, unfortunately, not going to meet these needs. Might the teacher be interested to join the free
Davidson Educators Guild?
While this may be uncomfortable, it is better than being in denial about your child's unique needs.

I don't know what I'm doing, because the challenges with DD were never this blatant or intense. I can't talk to other parents here regarding DD or DS abilities, because they turn so spiteful, like I've pushed this.
Yes, this is why we glom together here. Have you looked into testing and possibly applying for your DS5 to join
Davidson Young Scholars?
I know I will have to advocate harder, I know he wants math and I will push for him.
This roundup of
advocacy tips and threads may be of interest.
But is grade acceleration really a necessity? Or is there still a stigma?
This roundup of
acceleration anecdotes and resources may be of interest.