How do they know that a demon didn't take over the boys body?
Maybe that's why he was able to come back.
In real Christianity demons can't take over people.
The idea is that Jesus is the "good shepherd" and always around watching us and protecting us. Jesus would not let a demon harm someone. Jesus wouldn't let demons leave hell.
Jesus said that we aren't allowed to even think negative thoughts about each other, so he wouldn't be in heaven thinking that someone who's alive needs to be tortured.