Your son may laugh at you or think you're old-fashioned–but for his sake you need to take that risk and try to steer him away from something that will only hurt him in the long run–and hurt others as well.
The key, I believe, will be to let him know that you really love him–and because you love him, you don't want to see him do anything that will only harm him. God knew what He was doing when He commanded us to refrain from sexual relations outside of marriage, and we ignore His command at our peril. The Bible warns, "You may be sure that your sin will find you out" (Numbers 32:23).
Convincing your son may not be easy; he has grown up in a permissive culture that has thrown aside almost all sexual constraints. But we are paying the price for that folly, and not just in an epidemic of sexually transmitted diseases and unwanted pregnancies (many of which end in the tragedy of abortion). We are paying the price in emotional turmoil and insecurity and broken marriages.
Above all, I pray that you will urge your son to give his life to Christ. Yes, it would have been better if you'd talked with him sooner (and I hope your letter will encourage other parents to do this). But God can still work in his life, and this should be your prayer. Talk with your son because you love him–and God loves him too.