Lots of people here really seem to have a predisposition to disbelieve this. Skepticism is fine, because you don't know me or my son. I know my son, so I know that doubting what he says would be foolish.
I find your story HIGHLY unlikely. The only way a student can be thrown out of a class is if they are disruptive. Extremely disruptive. We're talking physical violence, or shouting and not allowing the class to proceed for a long time.
Children aren't stupid, and they're very manipulative.
The very fact that you are on this forum promoting this anecdote as some example of evil liberal professors will not be unknown to your child. He knows that this is going to be your reaction, that you have allowed preconceived notions to blind you from proper skepticism. So he's worded his version of the events to reinforce your behavior.
The red flag was when you said he made one mild statement, and the professor few into a rage and then he shut up. Come on, let's get real here. Nobody is going to fly into a rage because of one comment. Especially not a teacher who just asked for open comment.
What you just described is an event which will hit the newspapers. So while I'm very skeptical, I'm more then willing to believe your version of the events if you can send us a link to the school paper discussing this in more detail.