Have you talked with your son about the idea? I think a lot would depend on his comfort level. The fact that this district actually sought your son out and is already agreeing on acceleration and is offering to be flexible is pretty amazing. Many of us on this board are in districts who would love to have us disappear so that they don't have to do things out of the norm. I think it would be hard to compare your situation where the district actually *wants* to work with you to one where the district does not really like making accomodations for gifted children.
