What a shame that the children were so cruel and the R “Y didn’t have the common sense and know how to deal with the situation. Maybe the concept of karate now that he is older might not be a bad option to pursue again. It is not as if he needs the approval of his school friends or needs them to join along with him. Mon the other hand it might be considered cool among his new friends or at least for himself. He is allowed to do something for himself without asking permission from his friends.
If he has any talents he would like to pursue that might be another avenue to explore.