My friends were in that situation - a bright student suddenly stopped doing easy addition. I have never been in that boat,so no easy answeres here. For us things became easier when my DS started 5th grade and they allowed him to do Algebra. He was doing two classes of math daily - Algebra and his regular 5th grade curriculum - the fact that he was able to understand more complex problems did not mean that he instantly knew how to compute fractions :-)
Would private testing be an answwr, can psychologist talk to your son and then againt to the teacher/principal?
Good Luck,
Ania
