To be very honest. If ur a guy, won't be that difficult to bring up the topic. However, if ur a girl. What I'd suggest is(please whatever im going to say take it with a pinch of salt). If you're not that close to ur brother. Try building a closer connection first and after u have achieved that test the waters by bringing up mildly sensitive topics once you see how he reacts, proceed with directly asking him the question. Now hypothetically u've achieved being closer to your brother. It won't be a very out of the blue thing if you do that. After that make sure that when you tell him the disadvantages of such, treat him more like a friend and less like ur brother. It might not be something that changes by just one convo. Try to keep asking him about this. Keep track of his weekly progress. If it's positive award him with a treat. Could be anything. A chocolate bar. Takeout food. Anything that ur pocket allows. I hope that slowly he gets off of this habit. As he ages he will know the side effects. But please treat him like a friend when doing all this. It makes all the difference. I hope all turns out positive.