You may not know where your son is–but God does know, and God cares even more about him than you do. The Psalmist asked, "Where can I flee from your presence? ... If I make my bed in the depths, you are there" (Psalm 139:7-8).
This is why prayer is your most important weapon right now (and would be even if you did know where your son was). Only God can touch his heart and change him, and only God can help him overcome this powerful and terrible addiction. The Bible tells us to "pray for each other so that you may be healed. The prayer of a righteous man is powerful and effective" (James 5:16).
Pray not only that he will have a desire to fight this evil, but also that he will open his life to Christ. That doesn't mean treatment programs are useless; God certainly uses them to help many people. But when Christ lives within us, we have a new purpose in life, and new strength to resist temptation. Pray this will happen to your son.
My heart goes out to you and to all who suffer the agony of watching a loved one destroy themselves through alcohol or drugs. As millions of students head to college this fall, I pray that God will use your letter to warn them of these dangers and encourage them to turn to Christ for the moral and spiritual strength they need.