"No" to everyone

We seem to be in a phase. Any time another child or adultsgets within five feet of my son, he starts saying "No" to them. He wants them to back away. It doesn't matter where we are or the situation. He's always been Mr. Social so I'm not sure what's prompted him to start this. When he says No, I say Yes and tell him he's to share XX, to say hi or something else positive. Any other suggestions?

Thanks!