So, you're saying that you will have to home school your son because he won't be challenged in public school in the grade he is socially ready for, correct?
Depending on your state, school may not be mandatory right away. I'd just continue on the way you are and, like Anna said, give him access to the next thing - especially in areas he is interested in. Beginning formal schooling, with curriculum and a plan, need not start until he's around 8, imo. If he's a self-learner, interested in learning new things, he may go far before needing a formal setting. At some point he WILL need to learn to "sit down" and do things that do not come easy to him so he can learn how to endure.
I didn't start homeschooling until my oldest was going into jr hi. My second oldest was one of those kids that people liked to throw numbers at for him to add in his head as he was so good at it. He is still very smart and very good with numbers but school was never a "problem" for him. He just found things to do outside of school that challenged him.