Keep your cool in the meeting no matter what happens. Stay professional and focused. You mentioned that the director of the school implied that your sons needs would be met at the school. If you are not satisfied with the meeting with the teacher, I would go see the director (without the teacher if I could). Eventually you may end up with the teacher and the director together. I also agree with those that say an online math program, plus maybe some tutoring from a college student might be a good option.
If there is a school psychologist or counselor (who might be experienced in looking at results of your tests), it might help to bring them into the discussion as well. Remember that teachers (and directors) often have no more training than we do to interpret the results of these tests.
