Eighth grade was THE most miserable time of my youth. Mean boys AND girls.
It's tricky when it's the girls picking on the boys. Of course you'd like to protect him from all that, and it's a shame that the mob mentality of kids that age can be so vicious.
My own thought is that as long as there's no physical abuse, it might be good for him to learn ways of coping with it in a way that makes him feel OK about himself. As unpleasant as it was for me to be picked on, it did a couple of positive things for me: it made me appreciate the hurtfulness of words, making me more sensitive to the feelings of others, and it toughened me up a little.
Does the school have an anti-bullying campaign in place? If not, this might be a good way to raise consciousness. A meeting with the principal might be useful. Most educators these days are aware of the potential liability that can come as a result of allowing students to be bullied or harassed.