There is nothing scary I believe. The matter here is not to punish him and prohibit smoking. Everything because during puberty, teens are tend to do something that will be controversial to the words of their parents. They are real rebel during puberty and they want to boast their rebellion to their friends. I think that the best option here is to understand that it is his choice and accept it. Of course you can ask your brother why did he decide to start smoking, but you won't hear in respond something unique for you, I guess the reason will be "I saw my friends smoking and decide to smoke too". It's just a puberty, don't pay such an attention to his behavior.