You don't really...
Are you the adult responsible, or is he and his father staying with you for a bit? If you are the one in charge of him, then you can take disciplinary action. Otherwise, you need to let the adult that IS in charge of him know that certain behavior from the nephew is part of the stipulation of them staying there.
Discuss with the adult first that you appreciate not being lied to, and that certain house rules be followed. Then lay out the house rules.
If you get one on one time with your nephew, then be sure to share the importance of making the right decisions, pray for them, and let your witness stand firm, but loving.
Oh, and if the nephew is a young adult living with you, I hadn't thought of that.... then you can set down house rules and make sure that telling the truth and not hiding things be one of the rules. Make it clear, he's an adult that makes his own choices, but that if he chooses to live there, then he chooses to accept those rules.