When it comes to acceleration, self-advocacy was the only thing that was effective for my son. I live in Tiger Parent Territory and every parent in my area seemingly has a HG/PG child who requires a few grade levels worth of acceleration. The admins have heard it all and know how to push back effectively. But, when a child speaks up about their preexisting knowledge, asks insightful questions in class room discussions, enthusiastically throws themselves into open ended challenges, asks the teacher for work that is more challenging, then, they are willing to consider that the child is interested rather than it being parent-led interest.
My child got harder work (multi year acceleration) based on his self-advocacy (finishing his work quickly and asking for harder work to do in the leftover time every single day), bringing above level books to read in free reading time, working on very challenging projects for his free choice assignments, asking the teacher explicitly for challenging assignments, sometimes even asking the teacher if he would be allowed to sit in the higher grade room with older kids etc.
I would like to add that I had advocated in the past (different school) with less than stellar results
Self-advocacy is very effective for a child interested in "challenging" academics and age 6-8 is a great time because children at that age cannot be coached by a parent to say the things that they say and the self-advocacy comes out as spontaneous, genuine and earnest. My only advise is to let your son know that formal meetings with the principal are not necessary as a first step - he can just talk to the teacher about it while he also tries to demonstrate his grasp of subject matter. Most teachers are willing to listen to 6 year olds who have requests for more difficult work
