When your son comes to you for information about sex, don't get uncomfortable, dismissive or judgmental. Studies show that most teens want to get information about sexuality from their parents. So take advantage of this great opportunity to help guide your teenage son's sexual development.
As he's talking, listen for any hidden issues, concerns or feelings that he might have trouble expressing directly, but don't over analyze it. If he asks a pointed, specific question - - answer it. You don't need to start a rambling monologue on a wide range of sexual issues; he won't be listening once you get off his topic anyway. However, also make sure you don't avoid his question by not really answering it. When you do answer, keep in mind that simple, direct language will be most effective. Once you've answered his question, ask him to repeat his version of your answer back to you to ensure he's really understood what you've told him. Your last practical step should be to ask him right then if he has any other questions he'd like to ask. Sometimes, the first question and answer open the door to more dialogue.
Most importantly, don't wait for your son to open the topic with you. In fact, if you don't let him know first that he can come to you with sexual questions, he might never know he can talk to you about it. It's best to raise the issue of talking about sex before he's likely to be sexually active. Look for a natural opportunity; a good time might be in response to a sexual issue presented on a television show. Again, this first talk isn't a comprehensive study on sex. You might not even talk about a specific issue at this time. You just want to make sure your son knows he can come to you.