My cousin is a flake! Please help?

I'm 13 and I have a 12 year old boy cousin. We are very close. He's like a brother and a BFF to me. But he's started to not want to do anything. During the summer, he begged my grandparents to sign him up for tae kwon do, but now he recently quit because he "didn't want to do it anymore". He's become more irresponsible and more immature. He's missed a lot of his CCD classes and when he does go, he sulks. One time, I even had to go w/ him to CCD just so he'd go. Now his younger brothers have started doing the same thing. How can I convince him to care?