It's hard to say and if you ask him, he will probably not know. Some people see the devil as the perfect super villain. Perhaps your son feels powerless over some aspect of his life and wants to emulate someone who seems to have limitless power to get his way all the time. Perhaps your son has despaired about ever reaching heaven and he is trying to cover up his fear by identifying with the devil and looking like he doesn't care. Perhaps he is really into something demonic. Teens identify with goth themes and scary things as a phase. So, I'd say, first pray. Then go to a Christian book site for some helpful books, and schedule a talk with your pastor and/or a mature Christian friend. Then in a non-threatening way talk to your son to find out what's going on.
Children are very impressionable. If they don't have the right guidance at a young age, they're more likely to go in a bad direction. You cannot plop your children in front of the TV and expect them to raise themselves.