I went through this with my daughter last year, at exactly this time. There were a lot of factors involved: I was not impressed with the teacher either. She was not a good match for my daughter (the principal approached me at the end of the year and said as much). My daughter was bored to tears because like your son, she is also very advanced for her age. I think a different teacher could have dealt with that despite the course material. She was placed with a terrific teacher this year who can see her strengths and capitalize on them.
The other thing is, at this age they start to do a lot of testing, which is also very tedious and boring. Can't blame the kids for those feelings, and frankly the teachers hate it too.
My daughter started acting out on purpose and getting in trouble—which is so NOT her! After making several attempts with that teacher to spark my daughter with something challenging, I had to give up and just ask my daughter to give it until mid-March when testing was over and conferences were due. I told her I would reapproach the teacher then. By the time that came, she decided she could hang on another couple of months and just suck it up.
I just held my breathe after a while and was happy to see June come. Unfortunately, it happens and we won't always be so lucky to have a great match as we do this year!
Good luck...