Motivating Teenagers

Ok, I am talking on a personal level, which I believe I read somewhere is not proper etiquette, but what the heck. I cannot motivate my 15 year old son, seeing his last progress report brought me to tears. We all know the type, smart as whip, doesnt have to study, but can't seem to get that homework turned in. In class work, not completed, for no apparent reason, he is not talking. I've tried everything I can think of and have turned to my teacher toolbox more than one. He was down to just reading books with all electronics removed but he loves to read and gets lost in the a book (just like his mom). My question is this. How am I suppose to motivate a room full of students when I can't motivate the child I raised. I am at a loss, and I don't mind sharing.... If you have any advise, I'd love to hear it. How come 30 kids can sometimes be so much easier than the ones you make?