Could it be that he hasn't dealt with the loss of his mother to a degree that would've been necessary? Not only did he lose his mother but his father, too, in the sense that his father relinquished his parental rights and let your parents adopt him. Those are some huge issues to deal with. You mentioned that he was a brilliant and a conscientious student for the longest time until now. Why has he dropped the ball like that? Has anyone tried to talk to him about what might be bothering him? What kind of introspective ability does your cousin/adopted brother have? Is he showing signs of depression? A sense of meaninglessness causing him to lose interest in his life? What's going on? He doesn't sound lazy. Talk to him. Don't just have him plaster his walls with posters with pep talk on them.