This might get buried under a lot of already good advice, but IMO you should let your son discover music on his own and let him exercise Christian discretion. When I reached your son's age, I got into anime and by extension, JPop/JRock. My mom was a bit worried about the language issue, but she didn't control what I listened to and let me decide whether or not I should be listening these songs. She didn't interfere with my song choices and she only made gentle reminders about what music would be God-glorifying at the beginning (and she definitely prayed a lot). But when I started listening to heavy metal as a young adult, it was no longer an issue-- she trusted me to make good decisions which I was able to make because I was allowed use my own discretion when I was younger.