We are in a similar situation with DS12, 6th grade. We have not skipped any grades but he is taking Alg II this year with 8th graders.
He was an AIME qualifier this year and also made a 690 on the SAT math section, DS likes math and is pretty disciplined about studying. He self-studied the Art of Problem Solving's Intro series but I read that the online courses were more than just the book and signed him up to retake Intro to # Theory and he really likes it so we will probably also "redo" Counting/Probability and Geometry online also.
As you indicated, his acceleration in math will probably reduce his high school GPA a bit which I am trying to minimize as much as possible. As it stands, he will get credit on his transcripts only and his GPA will not be impacted although they will indicate his actual grade. He is making A's in all of the high school math courses and we initially thought it would be better to wait and take the Honors math courses to maximize his GPA but eventually decided to determine his math progression without thinking of the GPA impact first.
We were not aware of the AMC tests and AIME at the beginning of this school year and he has really enjoyed those and has learned a lot practicing for them. Art of Problem Solving is an amazing resource. Have you read The Calculus Trap, by Richard Rusczyk? It is a worthwhile read and we slowed DS down a bit after reading it. Good luck!!
