From personal experience, let me throw you a few pointers to keep in mind when approaching this situation. :]
•Keep in mind, if you didn't know about this or he didn't tell you about it, there is a reason.
•Remember, tobacco is addictive. If he has been using them, he is most -POSSIBLY- addicted. He may be, he may not be. Don't assume.
•He is family. Love him! Support him THROUGH your love. That's one of the best ways to let Christ help those around you-- IT CAN BE THROUGH YOU!
•Don't tell him smoking is bad for him. Smokers know this. Show him you love him and you want him around as long as possible.
•Ask him why he started, and show him love!
•If he is under-age, tell him he should ask his parents for help in dealing with it. (And for with advice on dealing with whatever made him start smoking!) I don't know a single parent who doesn't wanna be on good terms with their kid(s). Communication is a good way to help get on better terms.
•Don't play tattle-tale. (Unless he is really young, like not even 15 young). This ALWAYS ends up hurting the trust between siblings. It's up to him to go to your folks.
Most important thing?
•Pray.
Just talk to God. Like you were talking to your best friend. Because that's exactly what God is. Your best friend.
Ask Him to HELP YOU talk to your bro about it. Tell him what you think, and that you aren't sure what to say to him. God WANTS to help, you just gotta ask! It's all that easy! :]