I dont watch porn, and i am his age (13). I hear kids talk about it at school, and though it does make me curious to actually see it, I always try and back out.
I want to acomplish abstinence or chastity (same thing), i feel that porn might make me want to real thing. It might want your brother want it too. I would cut it off from him. Tell me that i know your curious but just wait until you are older.
And by the way abstinence is part of my religion, so it is required, but it is still extremmely difficult to go through, not that i am tempted to have sex, i just think its peer pressure that makes it feel hard to do.
Tell him to make a good group of friends, ones that are not porn liking, and tell him to try and keep away from porn and sex until he is at an older age.
I think my advise might help because it is comoming from a fellow 13 year old!