Joanna Fortune: Our teenage son has completely disengaged from us

"Teenagers experience a surge in development starting in early adolescence (13-15 years), peaking in mid-adolescence (15-17 years), and generally starting to settle in late adolescence (17-19 years)."
I suggest you start by reflecting deeply on what was happening before this change in his behaviour and what has been happening since. Could there be a trigger for this change?