Has he been tested? Do they recognize his abilities at all, or no?
If not, then you probably have to advocate with the teacher. This situation is a hard one for advocacy because many/most teachers do not realize that gifted kids often do worse on material that's too easy. You might use the "Please try giving him this more difficult thing...just try it and see how it goes." Sometimes seeing is believing.
Also talk to him about doing his best, even when it's easy work. Some kids are mature enough to handle that idea, some aren't. (Mine wasn't!) But it might help, especially if you explain that you're going to advocate for him, but that getting him more challenging work will be more likely to happen if he does well on the easy stuff. Plus the conversation tells the kid that a) he has to respect the teacher, but b) you're not just going to accept what's happening to your child without standing up for him. Those are two important lessons for him!
