Yeah Id want to beat the crap out of them myself. Bullying is a big thing where I live and kids commit suicide, its that bad.
I dont have any great advise for you and your son but these girls are probably doing this to get a reaction. You are right, boys would just have a fight and get it over with but girls Arghh! Im glad you can talk to your son about it, it would of been probably quite embarrassing for him, especially girls. I dunno, Im from the old school, girls or not Id probably advise my 3 sons to give it back harder. Im not into violence, sometimes bullies just need to know who they cant pick on.
All this, walk away from them and tell someone stuff - to me its just s---t
Ive always told my boys to stand up for themselves and it has worked so far.
Just my thoughts.