It could be about a lot of things. Maybe it's about a young boy's real - life experiences, his growth, his challenges, or some special events in his life. Without more context, it's hard to be more specific.
Yes, it is. The idea of basing it on a true story helps to set the mood. It allows the filmmakers to play on people's fears of the unknown that might be associated with real - life events. The true - story aspect can also make the audience more invested in the movie as they try to figure out which parts might have actually happened.
In most cases, we can't assume 'Boy' is based on a true story without clear indications from the creators or relevant background information. It could just be the imagination of the author.