Assertive but not mean

How do you teach your child to be assertive but not mean or rude? I don't want Jadyn to be mean or a rude obnoxious kid, but that at the same time, I don't want him to be a push over either. When we are at the park, he lets the other kids cut him in line, take the toy he was waiting for, etc. But at the same time, I don't want to try to change his personality.  He really is a sweet kid. His teachers say that he is the only boy in the class that doesn't bully the girls.  He (usually) is pretty good at sharing and making sure the other kids get their turn. He patiently waits in line until it's his turn (like at the water fountain or on a slide).

A little while back, we were at the mall playground and a kid pushed him out of the way of the slide.  He kept telling me that the boy pushed him.  My response was to push him back.  I know that isn't the right lesson either but what do you do? I am not the type of mom that will go to the kid (or the parent) and tell him to stop pushing. I think he needs to handle some things himself.