I agree its time to call the police but it would be difficult to prove they did this. Don't let your son play alone or go by himself to the park. Does he have some friends to play with? If not then you should accompany him. You can enroll him in karate, but it will take time to get the training and only use it when absolutely necessary. My almost 16 yr old is a black belt and when she was at a new middle school for 7 and 8, she was challenged to a few fights. She forewarned the girls she was trained in karate and they backed off. Karate would increase his assertiveness and confidence. Sorry this is happening.