Hopefully the bullying can be nipped in the bud.
My thought would be to talk with ds, making a list of positive qualities of his friend. From your post I've learned that his friend draws nice pictures.
Your son could answer a bully's statement with a compliment about his friend such as: She draws nice pictures.
Then help him make a list of good qualities about the bullies. Maybe one of them runs fast. Maybe one of them sings. This may help him see the bullies as regular kids with many attributes (including an occasional desire to mock).
Decide if you want to involve the teacher. Hopefully she will appreciate knowing what has been going on, and help mention compliments toward this student as needed. Each of the teacher's compliments to this student may need to be balanced with a compliment to another person, so that students do not see the girl as weak.
Authentic, well-deserved compliments lavished on all students in the class can help boost the morale of a classroom. Kids often begin to emulate the behavior they see role modeled, and may begin to speak in compliments to each other.
Some teachers have done a class exercise in which each student writes a positive statement about each other student. These are collected onto a page and kept. Sometimes they are decorated with artwork and borders. The teacher may read them to the class. There are stories with examples of this on the web. Sometimes lists of character traits are used. It depends on the situation.
