Hello my friend. Just to ease your worries, teen age boys will be rather quiet quit often. They simply are more into socializing with their friends, playing computer games, and on the internet. You must realize that your son is at an awkward stage of development. He is soon to be an adult and will leave his childhood days behind.
You are exactly correct as to how important communication is at all times but you do not want to seem like the overly inquisitive parent. All you can do is to ask your son questions that do not require simple yes and no answers. When he wants to express himself about something, he will. As for now, give him the space that he needs but make sure that you have a good idea of what he is involved in and who his friends are. A lot of the conversation that you have with him may be one sided with you doing most of the talking. However, that is perfectly fine as long as your son is continuing to be an active listener. Just make sure that most of the talking is not demanding your son to do things or yelling at him about something all of the time.
You are not alone in the way that you feel about your son's quiet and non chalant attitude. In time as he matures and develops, he will become more willing to talk with you. Do not take it personal right now!!!
If you need more advice, please ask. Thank you.