You mentioned home school. Can't hurt for a few months. If you homeschooled could you then put him back in a higher grade. How would schools determine what grade to put him in? Would they test or just go on his previous school grade?
This would be my recommendation. We did that very thing with a school district that refused to skip, and they took our DD back at the grade level we declared without comment.
I would also recommend talking to your DS about the importance of performing well on tests, to show what he knows. We had this same conversation with our DD, when she was rushing through homework and making obvious mistakes. "You know what you're capable of, and I know what you're capable of, but the school doesn't, and you have to show them, or they'll just go on thinking we're crazy, and that they're the ones who really know what's best for you. Show them what you can do, and then we can go back to them and say, 'See, she's past this, it's too easy, so skip her.'"
