How strict or mean should a karate sensei be?should they bully?

Most martial arts teachers can be very strict with their students, as motivation and discipline. But this teacher is definitely taking it way too far. If this were my child, I would confront him about it. If he refuses to lighten up, I would switch to someplace else. The way he is treating your son is unacceptable and will undoubtfully damage his self-esteem.