Don't worry - We all do it, right?
Don't worry about it. Everyone will find out about it someday maybe he came across it early. I came across my first pornography magazine when I was 7 years old. It's a natural thing.
You should sit your brother down and casually find out what he knows about the topic thus far. If some things are false, make an effort not to push him away by laughing but simply explain him how it all intertwines while maintaining efforts to not scare him away.