Holding your kid back because they'll be bored by the ponderous pace of public school education isn't the answer you're looking for. Continue to be an active parent and find them a different school or a program within the school where their above grade level reading is embraced. Also you don't always get to just choose what the kid gets interested in, they're going to focus on something and at that age you're just overjoyed if it's not destructive or expensive.
I had a similar problem in my elementary school in the 90s having learned reading early and pretty easily and even had a 2nd grade teacher get a little peeved because I was just reading ahead during group reading exercises and didn't know where the group was when it was my turn. The solution was getting the teacher to stop and the next year getting a better teacher and into a little group with the other good readers and tested for AIG early.
I had a similar problem in my elementary school in the 90s having learned reading early and pretty easily and even had a 2nd grade teacher get a little peeved because I was just reading ahead during group reading exercises and didn't know where the group was when it was my turn. The solution was getting the teacher to stop and the next year getting a better teacher and into a little group with the other good readers and tested for AIG early.