I may not be the best person to weigh in, as DS5 likes all sorts of stuff that we let him listen to that a lot of people would find inappropriate (e.g. he likes Black Sabbath, Danzig, etc. when it comes up in our music rotation
in amongst the Fleetwood Mac and Pixies). But I guess as long as you keep tabs on what he physically brings in the house, that's one avenue of entry covered. I'd be worried about the computer and MP3s more. For any computer he uses, you can regularly check for MP3 files, the presence of music-sharing software or links to sites where music sharing happens, etc. You can use software controls like Net Nanny (off the top of my head) to control sites he visits, restrict his level of user access on the computer to install programs, personally monitor his computer use, etc.
In the end, if he sneaks around he can probably at some point listen to things of which you wouldn't approve. I would just make sure the lines of communication are open and stay watchful, I guess.
