Most people understand that giving choices to children is important, choices that are more significant than choosing to wear a red shirt or a blue one. The purpose is to teach children how to take responsibility for their actions: telling him, for example, that if he chooses to procrastinate with his homework he is also choosing to have less time to watch T.V makes him accountable for his own choices.
A firm voice, the language used and a follow through are the important factors here. The tone is relevant because it should demonstrate that you're not angry, you are merely giving him a choice that's his to make. The language distinguishes between a true choice and a consequence - a consequence would be what you implement, a choice is what he makes and something you cannot be blamed for. A follow through of the statement would help him comprehend how his immediate choice impacted later events.
In all likelihood you will not be present when your son or daughter is offered his or her first cigarette; you can only hope that they will make the right choice in the moment. Teaching children to slow down, weigh the impact of their choices and make thoughtful decisions will help them become responsible teenagers. Especially, when you are not there to police them.