B.C.
Believe me, karate/taekwondo instructors know all about this.
No white belt starts class knowing what to do.
That's what they are there for - to learn and to practice.
If the bully is kicking him, you might want him to wear a cup under his clothes to school to protect him.
Try to keep working with the school/principal, supervisor and school board where the bullying is happening - threaten legal action and/or go to the media if you have to - that usually gets their attention. Document everything. No kid should have to be in a hostile environment at school. They should have some policies in place for the situation and they should be following them.