The most important advice you can give him is to seek God's will for his life. Perhaps he's like most people his age, constantly asking himself, "What do I want to do with my life?" But I hope instead he'll begin asking a different question: "What does God want to do with my life?"
After all, God created us; we aren't here by accident. And He created us for a purpose: To live for Him and to do His will. Is doing God's will a burden -- as some people seem to think? No! God loves us, and because He loves us He wants us to live life to the fullest -- which only comes from knowing God and doing His will. Jesus said, "I have come that they may have life, and have it to the full" (John 10:10).
How can your son discover God's will for his life? It begins with commitment -- a personal commitment to Jesus Christ as Savior and Lord. That means we turn from our sin and selfish ways, and put Christ first in our lives. If you or your son have never taken that step of commitment, ask Christ to come into your heart today.
Then encourage your son to ask God to show him the next step he needs to take -- and support him in his decision. God may not show him the whole path right now -- but He can be trusted to show him the next step. And remember: God's way is always best.