The truth is that you can't really do a lot about your friend smoking. You can tell him how bad it is but chances are he already knows it and it's just his way of coping with life - be it not a very healthy way.
You can tell him that you will not be around him when he smokes and that you don't want him smelling like smoke in your house (either for your own sake or because your parents wouldn't like it or whatever, doesn't matter) and things like that but I'm afraid you're just gonna have to accept it's something he does and you don't have to approve of it, and you can tell him every now and then that you don't.
I picked up the smoking habit around that age too, but I stopped for roughly two years ago. Don't ever start smoking tobacco!
I agree with The Giggle Blizzard. In my opinion the best way to help your friend is to let him know the concern you have about his smoking. If havnt already, really letting him know your feelings could help a lot. Im sure if he wont listen to you right now he will someday because you just want the best for your friend. There is only so much you can do until its best for you to give them their own space. I hope your friend can find recovery!