Only the smoker would know what would motivate them to quit. You can inform them the consequences about smoking all you want but if they don't seem compelled to do it intrinsically then there's nothing you can do. You can approach the subject as a concerned friend. 

If he knows what he needs to know about smoking, and I mean all the negative effects of it and how it affects his health, then you can just settle for reminding him the positive sides of quitting. It's a long process. It could take months to years before someone decides to quit even after constant encouragement. 

As the motivator, there is not much you can do when the person you want to help doesn't want to help themselves.