The most important thing you can do is to pray for him and his family. That may not seem like much–but it is, because only God can get his attention, and only God can change his heart and turn him away from the sin that has gripped his life.
Then go out of your way to let him know you love him, and because you love him you hate to see him make a wreck of his life. Right now, he doesn't want to admit he is on the wrong path; his pride and his stubbornness have blinded him to the truth. Ask God to help you confront him with the dangers of what he's doing–but to do so in a spirit of love and genuine concern. The Bible says that "Through patience a ruler can be persuaded, and a gentle tongue can break a bone" (Proverbs 25:15)–and with God's help, that can be true.
Help him too to realize that drinking never solves anything. Like many people, he is trying to escape from his problems through alcohol–but in reality he is only making his situation worse. If he needs treatment for alcoholism, ask your doctor or pastor for treatment programs in your community that might help him.
Most of all, urge him to turn to Christ and open his heart to Him. Only God can change his heart, and only God can help him deal with his problems. With Christ the future can be different, because He can change even the most hopeless situation.