I agree smoking is bad for your health, and is generally a really bad idea, and it's good to point that out. But there is a flaw in the approach I describe. It treats children as not having reasons for the things they do.
In addition to offering advice about smoking, a parent should try to find out why his child wants to smoke. The best way to do this is usually to ask and then to listen without arguing or interrupting (just asking questions to get clarifications and elaborations). Don't worry if everything the child says is wrong. It's not going to kill him in the next 20 minutes, so just hear him out before you respond.
Once you know why your child wants to smoke it can make a big difference in how you react. At the minimum, you can give your child more useful advice. If he doesn't know smoking is bad for your health, then tell him all about that. If he read some pseudo-science saying it makes you smarter, then explain to him about proper science. If he thinks smoking makes him cool, then don't tell him about the health risks in detail, just mention them and then focus on discussing the coolness issue. And so on. This is pretty simple but a lot of parents get it wrong because think don't think of their child as a thinking human being who has genuine reasons for his actions, they just think of him as a simplistic partial human to be ordered about and controlled into doing the right things.
When the issue is something less clear cut than smoking, then it's even more important to find out what the child's reasoning is. Maybe he has a reason you've never heard of before. Then you'd need to think about that instead of just telling him all your standard arguments that don't engage with his idea. Maybe if you know what he wants you can suggest a better way to get it, and then he'll change his approach voluntarily. To have any real hope of getting a child to change his behavior by choice, which is always preferable, you have to think about things from his point of view and see what reasons he does and doesn't have behind a given decision.