since i don’t know you, this comment is not to indicate i don’t believe you. if you know for a fact that he rebbe is being abusive and that you are not exaggerating the facts due to emotion (could be both), you may want to tell the menahel that either:
1) Your son will apologize for chutzpah ONLY AFTER the rebbe apologizes for his abusive comments
or
2) your son will apologize for the chutzpah ONLY AFTER the rebbe admits to you what was said, and apologizes to you for the way your son was treated.
But don’t forget I’m from out of town so things work differently around here . . .
Hatzlocho, I feel your pain and I wish you a swift and complete yeshua for your son and you from this painful experience.