How To Re-engage A Grumpy Vacant Teenager

[BrumStress]

This is my first post - but in desperation I'm asking (it's not a new question) but any advice would be welcome.
My 15 year old son has turned into the classic 'Kevin'. From a friendly boy, this de-motivated, monosyllabic teenager has appeared. I know it's fairly normal (I think?), but I'm worried as he literally doesn't want to join in a solitary thing any more. Family holidays, trips out, eating at the table with me, shopping at weekends, seeing friends. It will be all I can do to get him back to school. Has anybody got any suggestions as to how to re-kindle him again. He doesn't appear depressed - just disconnected and unengaged in everything. Thank you for your help in advance.