Talked to DS' math teacher this morning. She was really upset. The school does have a "no tolerance" bullying policy in place and she promised the situation would be resolved today.
As my son didn't know the other boys name, she pulled out a class picture and had my son point him out. Once he had identified the offender, the teacher told me that they'd had problems in the past with the same kid and she was going to refer him to the principle. She apologized profusely to me and my son. She sat DS down and made him promise that if anything happens again, he'll come and talk to her.
So, i'm hoping this is the end of it for now at least. I'll have to give him some pointers on snappy comebacks!
Thanks to everyone for listening to me whine!