I think there is option #4 and option #5-
#4- Tell your DS to stay in the general proximity of an adult in charge. He doesn't need to tell or go stand right next to that person- but be easily within eyes and ears for a few recesses. After a few days, it is most likely (thought not for sure!) that the bully will get bored and move on. On the same note, he could as a teacher he trusts for a job that needs to be done. I've had students ask me for "detention" to up their cool factor and help them hide from bullies. Cleaning the white board or restacking class books :-)
#5- You could confront the other parent or ask for a parent meeting with the principal. Let them know that this is an ongoing problem and that if it continues, you will file a harassment claim with your school district. If it doesn't stop, then file the complaint.
