The most important things you can do are to pray for your son and then urge him to think carefully about what he is doing. The Bible says, "Whoever turns a sinner from the error of his way will save him from death and cover over a multitude of sins" (James 5:20).
We sometimes forget that when a son or daughter goes away to college, they suddenly have a new level of freedom—and sometimes they don't handle it wisely. They no longer have anyone watching over them or telling them what to do; now they make their own decisions. Often, however, they think only about the immediate moment and don't stop to consider the consequences of what they are doing. The Bible says, "The wisdom of the prudent is to give thought to their ways, but the folly of fools is deception" (Proverbs 14:8).
Let your son know you love him and that you care what happens to him—physically, mentally and spiritually. Encourage him not to give in to the pressures of the crowd or go along with what everyone else is doing or thinking, but to put his life in Christ's hands and follow His path. God put your son here for a purpose (just as He did all of us), and life's greatest joy comes from knowing God and doing His will. Don't give up on him, but ask God to work in his life and open his eyes to Christ's truth.