Relationships differ from adolescence to adulthood—which you probably know already, having gone through both yourself. However, adolescent relationships should still be treated with respect, particularly as intimacy increases. A supportive attitude will encourage your child to be more open, which in turn will give you opportunities to offer guidance on sexual decision-making.
Most parents will experience their child going through a variety of crushes and casual relationships, so it can be difficult to know when he has actually moved on to a more serious relationship—and everything that might entail, including physical and emotional intimacy. The best barometer you have to judge your child's relationship is conversation. Talk to him about his relationship, and try to ask questions that will illuminate the seriousness of the relationship. Your child might be hesitant or shy to speak at first, but the more you seem genuinely interested in his relationship—as opposed to anxious or judgemental—the more he will feel safe sharing these personal details.
For example, you might ask: "How long have you and Kate been together now? Almost a year, right? That's a long time. Do you ever talk about the future? Do you think you will try to apply to the same colleges? If you go to different schools, will you try to maintain a long-distance relationship, or do you think you will break up?"