How can I be a good influence?

This boy, who is basically my boyfriend, has a tough life to live. His mom died from drinking(she was drunk at the time) and his dad and stepmom are now going thru a divorce at the moment. His moms side of the family is the party type, they love to drink and get drunk . They actually encourage him to drink, and he's 15 by the way. He is such a sweet kid and is good, but I want to help him stay that way, i know I can't make te decisions for him but I would like to lead him to make good decisions in the future, I want to know how I can be a good influence even if I am not there when he's tempted by alchohol or etc. Thank you for your help in advance:)