People turn to alcohol and drugs for various reasons -- but one of the main ones is to escape from whatever is happening in their lives and causing them pain. This seems to be the case with your son.
In reality, of course, he isn't really escaping from anything; in fact, in the long run he's only making things worse. But at present he thinks drinking dulls his pain -- and that's one reason why it's hard to get him to stop. In time, he could become so dependent on alcohol that he becomes physically addicted, making it even harder to get through to him.
What can you do? First, pray for your son. You might not be able to get through to Him -- but God can. The Bible says, "The prayer of a righteous man is powerful and effective" (James 5:16). Don't just pray that he'll stop drinking; pray also that he will realize what may happen if he doesn't change, and that he will face his problems and seek God's help to overcome them. Pray most of all that he will open his life to Christ's transforming power. Remember: God loves him even more than you do.
Then let him know that the only reason you have confronted him is because you love him and care what happens to him. As a footnote, your pastor or doctor may be able to point you to resources in your community that can help your son. God bless you.