A few thoughts here -
1 There was a substitute teacher. When difficult child 3 was that age, the worst problems happened with a substitute. Part of the problem was the unexpected change, part of the problem was having an inexperienced person with my difficult child. A bad combination.
2) Your email to the principal was a good one. You made some constructive suggestions, which always goes down well. it shows that you are trying to be part of the solution, rather than an existing part of the problem. it also shows that your aim is to work together with the school to resolve this.
3) Your son needs further evaluation. This is not necessarily a vote of no confidence in the original diagnosis - it was the best diagnosis possible under the circumstances and at his age at the time, perhaps. Any doctor so insecure as to feel threatened by this request is a doctor whose ego is more important than his patients' welfare. Use tat line, see what happens. If they blow up, move on. If they say, "Fair enough. Let's re-evaluate," stick around. For now.
At our local school, I wanted to change classes for easy child. The school refused, said that to move her, they would have to move another child and this could be damaging and unfair to the other child. They also told me no other child wanted to move (from the class I wanted easy child moved into) - turned out to be a lie. Then they said that the placements were made after the previous year's teacher agonised over the decision; I said that it was nice that she cared enough to agonise over the decision, but it did not, sadly, make her choices infallible. The principal told the teacher, who was very angry with me for this. Thankfully we resolved the differences. easy child's later progress proved me right but sadly, too much delay was caused in her education by the school's stalling and mucking around.
If you fight this, as I fought for easy child, they will fight even harder back. To a point. Eventually, if you can hold out long enough, they will cave suddenly and completely. But in the meantime it can be messy, painful and damaging. One more important reason to protect your identity here - the local school tracked me for a while, trying to find everything I was writing. For example, I had sent in a letter to a gifted & talented students magazine (a publication for parents) and they published it; a teacher at the local school passed copies around the staffroom. If I'd known they would publish, I would have asked for a pseudonym.
Watch your back, but this is your child. He deserves some answers and some help.
Marg