I registered just to post this. My situation as a young kid was virtually exactly analogous to your son's. I always excelled at math. An example from my youth: I watched Square One
, and my parents told me that when I was about to start kindergarten, I asked my mom if/when she thought the teacher would be covering the Fibonacci Sequence. I have a worksheet that my mom saved--on the front, there were simple addition problems on the front, and on the back, I was doing multiplication problems of my own creation (the best one on there was 598*2). I don't have a large memory of how I felt at the time while I was in school, but by the second grade I was apparently pretty bored in my classes. I don't know the exact sequence of events, I was placed in a 4th grade level math class and I also started participating in a gifted program at the school. Putting me in that 4th grade math class was a blessing. I would have gone crazy if I hadn't been able to go into that program. I took calculus in high school, and I excelled in it.
I would talk to your school administrators about possible programs like this for your son. It doesn't have to be skipping grades or anything like that. Even though I could have been bumped up a couple grades, my parents kept me with my peers, and for that I'm thankful. I think that the programs are on a state by state and possibly county by county. Research options for your location, and try to be nice to the administration, however obtuse they may seem to be in the handling of your son's case. Unfortunately, they have a lot of control on the options available to your son, so try to approach the situation with grace and understanding.