You might consider calling the school counselor (if you have one) and ask her for some tips you can suggest to him to help him deal with it.
What's aggravating about the whole thing is that if you (general you) don't 'tell'...the kids doing it 'win'. JMO I've struggled with this as a parent, and it's so frustrating. You honor your child's wishes, and don't talk to the teacher or other parent and struggle with trying to figure out how to deal with it on your end. What is wrong with that picture!? Ugh! Sorry...my frustration coming out. LOL
I'm trying to remember all what the counselor told me...I have encouraged my son to talk back, but I don't encourage him to say mean things, and he wouldn't want to do that anyhow.
So he says things like "So?!?" in an annoyed voice. Or "what exactly is your point?" Or he'll imitate the person in a tiny annoying voice...you know, repeating exactly what the person says. LOL How annoying is that?! When someone is repeating. LOL! But that doesn't work with comments about someone's body.
He could say something "THANK YOU! You're SO KIND!" in a sarcastic voice.
He could say "blah blah blah" and just walk away. Stick his tongue out. Cross his eyes. Say something like "can't you think of anything else to say?" etc
Do you have other kids? I brainstormed with my younger son about what annoying things he did to his older brother (and vice versa) and how he could use those "tools" to deal with obnoxious kids.
My younger son told me once about a little girl in his class that just started saying REALLY LOUD, when kids were teasing her "STOP TELLING ME I'M UGLY!". The teacher knew about it immediately, and took care of the kids doing the teasing, but the kids in a way - outed themselves...kwim? They weren't anticipating that she would do that. The other kids were making fun of her glasses, saying she was ugly.
For my son, for a long time, he never said a peep. When he finally got the courage to talk back, the first time, he was SO empowered! He came home with a big smile on his face! So for him, he felt like he was taking charge of the situation and not being a victim to it.
I'm a bit of a pain as a parent, because if I find out who is doing the teasing, I might not be able to control myself when I'm near the parent. LOL
We had one kid teasing my son, and I found out she was on our swim team. I was standing with other parents, and my son indicated to me, that's Ally, she's the one that said _________ and _______ to me. And it went on for MONTHS. So the next time I was waiting with other parents, the situation just presented itself when the Mom introduced herself to me and introduced the child to me too! LOL! I said "Oh! It's good to finally meet you! DS has been telling me how you make fun of him every day on the bus! You know, how you say _______ and ______. It's really been such a joy meeting you!" and I walked away. She hasn't made fun of DS since. Needless to say the Mom doesn't go out of her way to talk to me either. I'm okay with that! LOL! Now, that could have totally backfired on me too. I'm not usually confrontational, but sometimes, as a mother...you can't help yourself.
Teasing/bullying has been around since the beginning of time. It will never go away, despite the good intentions of parents/teachers etc It's a really unfortunate part of life. Helping your kid deal with it, is so hard. I'm sorry this is happening to you and your son.