you said he was fifteen. If you mock him saying he's gay for not liking porn (no offense to the homosexual community, it just works on fifteen year olds) it might work.
actually i was joking there but really i agree with some of the other posts. get proof of him watching it too and blackmail him. that's what brothers are for.
you also said you didn't know exactly what he had on you. so if you blackmail him and he doesn't know, you're now a leg up.