Have you talked to the teacher about this yet? I wonder if they have some flexibility to give other curriculum to kids whose needs aren't being met. I know that my son's school adopted a new literacy program last year, and all the classes were supposed to use it, including my son's HG program. The teachers do have some flexibility though, and I think they used it for a month and then set it aside. A lot of reading programs are not geared toward gifted kids, unfortunately.
I recommend first talking with the teacher, explaining how last year was so successful when your child got to select his own materials. If you get the sense that the teacher doesn't feel there's any flexibility, I would bring in the curriculum director/GT coordinator/principal for a powwow about what to do for the kids who are beyond this program.
