I think you've gotten some great advice already. The thing I'm wondering is whether you know what level the "challenge" work is and what type of work it is. It could be that it's still not really challenging. Or that it's busy-work instead of really learning anything, or that your son doesn't want to work independently. I know my son likes doing some reading of new topics on his own, but he'd rarely want to do a worksheet or such on the same topic. Worksheets tend to be very repetitive. And what he'd really like most of all is to read or learn about a new topic *with* someone and be able to ask questions and discuss. That's his preferred learning style. Perhaps from your homeschooling experience you already know what works best with your son, or you automatically are using that method without thinking about it? Perhaps his teachers who know so little about him are not.
Another question to ask yourself is what you're hoping to get out of sending him to this school IF they can make the IEP work? Is it worth fighting for the IEP and advocating again and again to get the right work in order to get the benefits of the school? Or will the battles with the school negate some of the benefits it might have?
And to plant a slightly devious seed -- what would happen if you pulled him from school until the IEP is resolved? Oh, sorry, they'd probably call you manipulative and call in a psychologist.
Though I really find the idea of calling a 6 yo "manipulative" humorous and appalling, IMHO, a psychologist could be helpful if it's someone is familiar with needs of the gifted and can give you and your son greater insight into the workings of his mind and how to deal with these problems. Find out what experience the person brought in by the school has and consider that in their recommendations.
