First, a disclaimer, I am a teacher myself, so I am coming from her perspective (sort of).
1) Volunteer in the classroom, but don't just watch her and what she does, watch the students as well. See how other students react to her. That will be your best guide as to what is really happening.
2) You started your post with "My son doesn't like his teacher." If he doesn't like her, in his mind, the best way to get out of her class is to complain. I am in NO WAY saying your son isn't telling the truth, but there are always 3 sides to every story - Person A, Person B, and the truth, which lies somewhere in the middle. Teachers have to be very careful about what is said and done these days in schools. It may have been a situation where your son was talked to about something early in the year, and then he decided he didn't like her because of that one incident. I guarantee you, if she is saying those types of things to an entire class, she wouldn't be employed any more, at least not without constant monitoring and a very specific growth plan in place. There are so many young recent graduates with teaching degrees who can't find jobs, there are more than enough people to take her place. And, as far as it goes for tenure, that depends on the state, but in CA there is no tenure until the college level. Here in TX it is the same thing. Even if a teacher has been employed for 20+ years, a district will not put up with things like this.
3) The fact that they are refusing to move him is telling: she must be a good teacher. They don't keep kids in classrooms that arean't good just because it is a hassle, they do what is best for the student.
4) I have told parents that a conference was over, not because I didn't like what they were saying, but because the next time I met with them I wanted to have a counselor or administrator present, or because classes were about to begin, or because I had to leave to pick up my own kids, or because I had to go to the bathroom. There are a million reasons for that.... did you ask her why it was over so abruptly?
5) Ask for a meeting with your son, the teacher, and a counselor and clear the air. Counselors are the best advocates in these cases. They hear lots of things that happen in classrooms, and may be able to offer some insight.
Good luck in this tough situation.