Everyone has given good, practical advice and I can't really add more but I am so sad and angry for you. Every kid deserves to feel safe at school and anywhere else they go and the school is seriously failing your poor little man. I won't even start on what kind of parents raise a child capable of being such a bully.

My only advice to you at home is to let your son see that you are angry (if you're not already). He needs to know that this is absolutely nothing to do with him, but is ALL to do with the other kid. I hate the thought of your DS feeling less of himself because of it. I want to give him a big hug now