It's an amazing thing to do. It's such a shame kids are not understanding, all he can do is say why he's doing it and that he is OK with the long hair. Could you speak with the school and they could do something celebrating it? He would regret stopping after coming so far and the feeling of achieving his goal will outweigh the horrible comments. He's helping others in memory of his nan, that's such a wonderful thing to do. Maybe go onto Princess Trust website and remind him of the good it does