When my son was 7, I had a number of fruitless discussions with his teachers about allowing him to advance in mathematics. They made some promises about "differentiation" but never followed through on them. I brought them a completed fourth grade math workbook and a half-completed fifth-grade math book, and they really didn't even look at them. Given what you wrote, I suspect your son's teachers aren't going out of their way to help him?
Here's the thing. Many elementary ed.-level teachers in the US just don't understand math. Given this fact, they're heavily dependent on what the book says as a way of assessing your child's knowledge. So say the school's first grade math book shows 3 methods of doing addition (say, counting things that you combined, jumping forward on a grid or a number line, and a rule-in-rule-out algorithm).*
Now say that your child gets the Common Core "addition means combining things" idea intuitively. If the teacher doesn't honestly understand that those 3 EM methods are just different ways of showing this one concept, she might decide that your son's understanding of addition is incomplete. She might especially feel that way if he starts getting frustrated with rule-in-rule-out. To the HG+ kid, it's lame. To the teacher, it's critically important because the book said so. The book was written by experts!
My best advice to you is to 1) teach him yourself at home or 2) hire a tutor. I've been teaching math to my kids for several years, and it's been very effective. One of my kids also goes to the local Mathnasium, and they're also wonderful.
NOTES:
1. If you go the tutoring route and have a Mathnasium in your area, check it out carefully. Will they let a child work on material for older kids? Policies differ between locations.
2. DO NOT USE KUMON. They're about getting 100% of problems correct in <[insert number of minutes]. If you get one wrong or don't get one done in the allotted time, you don't advance.
*All from 1st grade Everyday Mathematics/EM.
