The head instructor definitely should be made aware -- but it might not hurt for an older person to talk to the kid before or after class. I strongly suspect that he's never really been taught how to teach, and there are probably two things going. First, he's probably mimicking someone who taught him -- but also probably going beyond what that person did. Which actually sets up the second thing; if he's 18 or 19, and suddenly teaching a class full of adults, some or many of whom are old enough to be his parents... he's probably less than confident, and hiding it behind the mocking. Either a senior black belt, or even a senior student, can discuss this with him privately, and may be able to stop it.