Sorry to hear about your sons troubles at school. Is there any way that perhaps they might let you send in some stuff for him to do or perhaps use an external source for his gifted ed? Something like, EPGY, perhaps?

I have heard so many times about how some schools just don't get the difference between more challenging work and busy work. It makes sense that busy work would drive your son to distraction. I hope your son feels better about it all soon.