Ok. I will start with my limited experience. Dealing with bullying kids is tricky. We can not encourage our kids to revolt and beat them back. All you have to do is make your son brave, by spending good time with him if possible try to engage him some sports training. By doing this you are eliminating the dependency of your kid to play with those indiscipline kids. your son will overcome the fear (which is more important)
Try to present in the scene, when your kid playing with them, that way you can control those bullies. STAY STRONG.