P.B.
My son is always the nice kid. Always sharing, always moving out of the way, etc. Finally, I taught him to speak up. Congrats!!! Your son already does. Its so hard to teach such good manners to our kids and watch while other parents do nothing. Depending on the behavior of the other child, if its really abusive, I do say something to the child "excuse me, but I believe my son was playing with that toy first, and I'm sure when he's done, he'll be happy to give you a turn, right?"
My son is now 6 and there was a 3-yr-old at the park pushing him out of the way and throwing dirt at him. My son threw his hands up in the air...I went after the 3-yr-old, but he ran away because he knew he was being bad. I told my son, if that boy hits him, pushes him, or throws anymore sand at him, to tell him to stop it and feel free to defend himself. My son looked at me like I was crazy. =>