Your son sounds a lot like mine! We are lucky enough to have an AoPS Academy in town, where he takes in-person math classes on Saturdays, but I would take a look at their Beast Academy classes online.

I had success getting him into the gifted program via a petition that argued that his autism didn't allow him to demonstrate his verbal skills adequately on the CoGAT. aeh was extremely helpful for helping me find documentation for that. Here is the thread.

Good luck!