I have a question about my nephew?

ask him why he's doing what he's doing. stay calm and don't start off by yelling at him or he'll just block you out. i used to smoke when i was 13 and 14 because i was under a ton of stress and i also started rebelling like he is. he could just be trying out the whole rebellion thing because his friend's are or because it's just fun or there could be a reason why. try to find out what it is. and he may actually be more willing to listen to you since you're not his mom.