N.S.
I don't have experience with special education, but I do know that teachers shouldn't talk down to students let alone single them out. I am a mom who never put up with that sort of treatment without a justifiable reason. With that said, rest assured, if my opinion means anything, your concerns are valid and warranted.
However, because the information was second hand, the teacher is leaving the program, and you only have a few weeks left before your son starts a new program, I'd just ride it out. The fact that the teacher is leaving is probably a sign that either the school, the teacher herself, or both have concluded that she's not a good fit and is better off in a new setting. So that can be a good thing. Because if the school would support someone like that, that's almost worse than dealing with one bad apple in the cart...then you'd be dealing with a bad batch of apples.
From here on out, I'd look at this as a learning experience. Perhaps put together a check list of assessments or requirements you need met for you to feel your son is getting the education he needs, rather than relying solely on the opinion or recommendation of others about a program. I think awards or rave reviews only go so far where schools and programs are concerned. I'm a firm believer that "mother's intuition" trumps everything when it comes to making major decisions. I say meet the teachers he'd be with and have a casual coffee clutch with them.
Once school starts, do regular unplanned pop-ins, and schedule regular visits or make phone calls to check on his progress, all the while checking on the staff. This at the very least will put them on notice that you are involved in your son's well-being. If they have a tendency to slip you'll be sure to catch it, and at the very least they'll work harder at trying to meet you and your son's needs.
Last but not least, keep tabs with your son. If you sense he's not happy, or he says he doesn't like someone or something about school, be his biggest advocate and take action, even if it's quiet observation or actively filing a complaint after getting hard facts of your own.