My Son was the same, and although he has ability he did as little as possible to get through and stay out of trouble. Leading up to his exams, I talked about the day in August when the results are out and everyone is buzzing and pleased with their acheivements (his sister is a year older and he remembered everyone doing this), and how he might kick himself for not revising, if he finds he doesnt do as well as he wanted. I offered money in increments for each grade he could get from C to A* (he was predicted passes in everything so I wasn't setting him up to fail) He did knuckle down more (not really enough but more!) and he got 2 A* 1 A 6 Bs and a C. He was chuffed with this, and we were really pleased for him. If you mean generally rather than for exams. I would ask him what he sees is his desired lifestyle when he's older. If he says own house, car, holidays, family etc then mention how much this might cost, and what he should be aiming to earn, then what careers would give him this, then what results he needs to get to enter into that field or course. He might refocus his motivation. Good Luck to him.