Teen Mental Health: Sudden Social Withdrawal
It may be quite helpful to explore when your child began to shift from being outgoing to being withdrawn. Were there any significant events going on at the time of his withdrawal? Are there times when he isn't withdrawn?
Keep in mind that what may be insignificant to you or others may be extraordinarily significant to your son, and vice versa. That said, often someone withdraws when they assume that what is troubling or exciting to them would be misunderstood or dismissed by significant others.
You can be assured that outreach and withdrawal are intentional and are significant ways your son chooses to relate to others in specific situations. In trying to reach him, you may find it most helpful to talk with him rather than to him and to find ways to validate what matters to him. Trying to make someone talk (or listen) usually results in the opposite outcome. At the same time, ignoring withdrawal or isolation can reinforce the possible assumption that no one cares. I would suggest letting your son know that you have noticed a change in how he is relating and that you want to respect how he is trying to care for himself, but also that you are concerned for him.
Keep in mind, however, that solitude is not isolation and can be a way for your son to explore his own self- and other-awareness and understanding. If he avoids others continually, begins sleeping more and more and becomes more and more immobile, I would rule out any medical concerns by speaking with your child's doctor and then consider seeking counseling. Yet, even in that situation, being with him, attuned and attached, without being intrusive or authoritarian may clear space for further revelations. This comportment with him will communicate your respect for him, which in turn may result in his choice to be more open with you.
Finally, remember that none of us behaves the same way in every situation with every person—even at home.