Hi M.!
I hope I'm not compeltely off, and though my daughter is only 3, I've seen this happen with several other children. I think there is a tendency to check for the worst because we fear it may be the worst, sometimes. If councellors, doctors, etc don't seem to find anything wrong, AND you've received multiple opinions, then it's likely that everyone's just looked in the wrong places. My husband did the exact same thing when he was that age and as it turned out, his vision suddenly began a rapid decline so that he couldn't see properly. When he couldn't read something, color in the lines, etc, he would get incredibly frustrated. They get scared and frustrated. Another friend of mine experienced the same thing with her child and after all the drugs, tests, etc, they finally discovered he's dyslexic.
I don't know if you've looked into "other" possibilities such as those yet, but it doesn't seem that your son has a clinical, mental disability. Another thing to consider, has there been any significant changes in your lives just prior to his rapid change in behavior? Sometimes things that are easier for us adults to handle are not as easily adjusted to a child. I would also be cautious to take a friendly approach towards him rather than accusational or disapproving disciplinary actions. I don't think it is something your son is doing out of spite, since he's never been this way. Talk to him and tell him it is important that he does well in school and you're proud of him for trying but you know something is bothering him and you want to help.
I'm sure you're having a very difficult time and I hope I've helped in inspiring new ideas, at least! Good luck with your son!!