In all honesty, you probably can't convince your son that he's wrong right now - because he's probably not willing to admit that he might be. Hopefully, some day, he will be open to changing his beliefs - but right now, he isn't.
What, then, can you do? The first thing you can do is to let your son know you still love him. Frankly, arguing with him probably won't accomplish anything; it actually may close the door even tighter. The Bible says, "A gentle answer turns away wrath, but a harsh word stirs up anger" (Proverbs 15:1). Instead, let him know that nothing can destroy your love for him. And as you have occasion, remind him that God loves him even more than you do - even if he doesn't believe in God.
Then ask God to give you the patience to hear him out - to let him tell you what's been happening in his life since going to college. Challenge him also to face the fact that his real problem may be that he wants to run his own life, and he doesn't want God to interfere. Urge him, as well, to be honest about his doubts; if he is, he'll seek answers to them.
But the most important thing you can do is to pray for your son. College is often an unsettling time, and unbelief is more fashionable on many campuses than belief. But pray that God will break through his resistance, and show him not only that God exists, but that He loves him and wants to come into his life.