How can I help my really close friend stop smoking/drinking?

My friend is an alcoholic and smokes like a freight train. He has been smoking for almost 4 years and he just turned 18 which is even worse. Me and my friends think he smokes about 2 packs a day plus weed. He has been drinking for a much longer time. We are so worried because we are afraid he might not make it to graduation this year, or any year for that matter :'( I know we can never make him stop as it is his own choice but is there any way we can help him stop? He is one of my best friends and I don't know what I'd do without him.