My 17mo is an only child. Over the past few weeks he has turned into a bit of a playground bully. When he encounters another child, his response is typically to shriek at them or to scream "no no no!" aggressively in their face. He did this regularly to our friends' 28mo when we were visiting over the weekend. He has an electric train, and when he notices our cat sitting and watching the train he runs over and scares him away. Today he apparently pushed a little girl down the slide at the playground.

He is perfectly friendly and extroverted with adults, and is often outright flirtatious with strangers.

Is there anything I can/should do about this? Is this normal behavior that he will outgrow? It is really aggressive and makes me sad to watch.