There is a problem at the school if this is not being addressed and dealt with at that age group. I work in a public school and if I saw that going on or heard about it and investigated further and found it to be true, action would take place to make it stop.
I would email the teacher and ask to have a phone conference to discuss what is going on and to find out if the school is aware of it and what they are doing or plan to do about it (if you are making them aware of it).
Next, I would teach your son how to respond to a bully. Kids who bully look for a reaction from the person being bullied (tears, fear, that sort of thing). When the boys tease him about playing with the girl, he needs to look them in the eye and matter of factly tell them something like this: It seems like you are having a bad day today. The things you are saying are not nice. Normally I do enjoy talking to you, but not when you say mean things, so I am leaving but if your mood changes and you want to be nice again, feel free to let me know. Maybe we can play together later.
If he shows self confidence and no fear, the boys should stop picking on him.
