I can't believe they acted like that with you sitting there! I'm a bossy former teacher, so I'd probably give them a good tongue-lashing and a lesson about manners. But that isn't necessarily the right thing to do here. Just something I couldn't help doing...I detest bullying and I can't let it go by without comment.
I did read somewhere that some huge percentage of bullying (85% sticks in my head, but I could be way off there.) stops if someone--anyone--says something about it. For that reason, I'm not sure that ignoring it is a good idea. Maybe a quiet "That's not very nice" would help? So much depends upon your personality though...
Mostly I'm just sorry you and your son are going through this.
