Sorry to hear your son and you are having issues. Being a teacher today is a very thankless job and the reason why people even become teachers is to help, nurutre and guide todays youth.
Its a takes alot of pushing in order to get a professional to go against the very principles they believe in. Today's kids are much different than when I was growing up.
Kids today think respect is given and not earned. It was wrong of the teacher to say that, I am sure he is fully aware of the reprocussions.
Has your son apologized to the teacher for the misunderstanding and or how the situation should have ever happened? I believe assuming some responsibility is in order for everyone to move on..