Does your state have GIEP laws? I'm sorry if I'm missing something from previous statements in the post. If you have a GIEP, generally you can request a meeting to occur. You can drive the meeting, and you can refuse the accomodations they are offering if you feel they are not enough.

Pretests are usually reasonable IEP accomodations for Gifted children. The teacher cannot refuse to do something on the IEP. So, once it's in there, you can enforce it. Make sure the IEP also contains some kind of activity he can do while other kids are learning the topic at hand.