I'm going to chime in.
This happened with DS12 in 1st grade. He had the brash, militant teacher. They had to color in every drawing on every worksheet sent home, including the background. She never yelled at DS. He was quiet and shy. But she yelled at a lot of the other kids in class and that upset him. Your description of your son sounds very similar to what we were going through with DS.
I made an appointment early in September and spoke with her about DS' unhappiness at home, the crying, the stomachaches, etc. She coudn't understand why on Earth that would be happening, but was quick to show me how "disorganized" DS's work station was and let me know that he was not working up to her expectations. I thought maybe he was just struggling with the increased work load, talked to him about how we need to learn to work with different kinds of people, and let him know that we weren't concerned at home about his performance and we thought he was amazing, and etc.
A week later I got a packet of his work home. They give numeric grades even in 1st grade at our school. He had 100 on everything. EVERYTHING. From math to spelling.
I called the principal on that day, a Friday afternoon at 4:30, to leave a message to schedule a meeting. She picked up the phone. Emotional conversation ensued, and she offered to move DS to another class. Just like that.
Of course it took an additional 2 weeks to get it done, because the AP was best friends with DS' teacher and didn't want him moved, but we eventually, by documenting every little snippet DS shared about her, got it done. He was moved to the "nice" teacher, who, admittedly, did not challenge him, but he felt safe, and that was more important for his 6 year old self at the time.
We've never regretted moving him. We caught some flak from other parents at the school, but it was worth it. She was absolutely the wrong fit for DS., and IMHO any gifted child. He's never had a problem with another teacher and is still a strong student in Middle School.
If you haven't tried yet, it might be worth escalating to see if he can be moved to another teacher.
You mentioned testing. Actually, that was your question, and I answered in an entirely different manner. Sorry! I still have to sit down with teachers every year and go through ideas, and plans, and so forth but it's been helpful to have a baseline from which to work, a research based metric that no one can argue with. We've never done more than the basic school-provided testing, but for DS that's been all we have needed.
