As your teen looks for ways to become more independent, you might find that staying involved in his life is becoming a bigger challenge. Regardless of the obstacles, you can stay involved your teen's life through active engagement and meeting the other people in your teen's life.
Talking with your teen is, of course, the primary means of staying involved. While asking him questions about his life is important (and do keep doing that!), you also want to learn about his friends, activities and interests. Meet your teen's friends; find out where your teen goes with them, what they do together. In addition to just taking the smart step of knowing what your teen is up to, knowing this information can also help you plan activities for yourself and your teen. Getting your teen involved in activities he will really enjoy that you can participate in yourself can be a great way for you two to connect.
Also, set the example about the importance of the family knowing where everyone is and when they're due back. Once you've set a curfew for your teen, enforce it. Tell your teen where you're going and when you'll be back -- and then stick to it, or call! Taking this approach will help your teen see your questions about where he is going (and with whom, and when will he be back, etc.) more as good family policy rather than an attempt to clamp down and stifle his independence.
You can also talk to the other people involved in your teen's life, who can give you a different perspective on who your teen is. In addition to your teen's friends, talk to his teachers, coaches, counselors, etc. Any other adult who oversees your teen can tell you about his behavior, moods, interactions with other teens, and other valuable information that can keep you aware and involved in your teen's life.