Enjoining or commanding the good and forbidding the evil is a Muslim's duty. Sometimes, we are not in a position where we can do anything about pornography with our hand or by saying something about. The least we should do is hate this in our hearts. That means hating to hear dirty jokes, to see half-naked individuals selling products on television or in magazines, and hating to encounter this kind of material in general. By hating it, we will develop a sensitivity against it, and be able to see how disgusting it is.
Friendship is built on concern for others.
This is why so many young people turn to friends instead of parents in times of crisis because many parents often don't express this concern openly. By showing genuine concern, your advice is more likely to be heeded, Insha Allah. Don't try to change Imran so he can see how much you know, or to impress others. Do it sincerely, for the sake of Allah and because Imran is your brother, and all Muslims are like one body. Even if Imran doesn't realize this is wrong, belittling him or exposing this fault of his in front of others will make him defensive and angry.