I'm so sorry, Belle! Yes, it is painful to watch someone dismiss your child when they have no understanding of him. I think your suggestion of having the previous speech therapist call the new speech therapist is terrific, as well as bring in more advanced material. I don't know if you could speak to the new therapist alone for a few minutes before the next session and explain that you DS is very advanced, and that she should try to imagine that she is dealing with a 3rd grader instead of you average Kindergartener? With her attitude and her instant snap judgements, it may not make a difference.
I know that I have had to take my DS aside sometimes and try to explain that people do make snap judgements about people based on behavior, even though this leads to erroneous assumptions. We had a terrible time once, not too long ago, when DS had to get his blood tested for a latex allergy. The nurse took one look at DS, judged that he was anxious and uncooperative, and made the snap decision to call in another nurse, grab him and hold him down for the blood drawing. He screamed and thrashed and was furious over the entire event. We had a long discussion afterwards, while he was still shaking with fury, that people do make snap judgements and that you have about 15 seconds sometimes to convince someone of your willingness to cooperate. I then explained that snap judgements also are formed when the person feels overworked or pressured to move on to the next task. (We were sent to a pediatric urgent care for the blood drawing and the waiting room was packed!) I don't know if my eight year old absorbed this or not, and I don't know whether your five year old could, but it is something that these intelligent, sensitive kids will ultimately figure out. It might just take a very long time, at least for my little guy!