From what I remember the subscription sites operate on the model that the music isn't really yours. In that if you stop paying a monthly fee you no longer have access to the music. Obviously there are ways around this however. I think the benefit with these places is that you can grab an unlimited(?) amount of songs. It also seems like the DRM is a bit more restrictive.
Itunes operates on the .99 per song and 9.99 for an album, though I have seen the album price lower as well as higher for the bigger collections. Their DRM allows you to have 5(?) computers authorized for the same account and you can deauthorize one to allow another so no fear in running out. Though I've noticed it doesn't clear them all away for me (Apple might be able to help me on this but I haven't run out yet so it's no a high priority).
The problem you will run into is that their songs are in a modified form of AAC. There is a way to strip the songs
of their DRM and then convert to something more universal, like mp3. The problem with this, and the site says this as well, is that you need to install an earlier version of iTunes or it won't work (The stripping part). There may be other software solutions as well but I haven't looked. And if worse comes to worse you can burn the songs and then extract them from the CD. I haven't tried to convert a protected song but highly doubt it would work.
It looks like your player should be under the PlaysForSure label so these sites
should work the easiest for you.
My personal preference is of course iTunes. While I do own an iPod I'm sure that it only plays a part in the decision. Their interface is clean and purchasing is easy. Not to mention that if you are into podcasts they made subscribing to each one simple.
Could it be better? Of course. Sometimes the samples barely get any vocals so you don't really get a feel for a song. I've come across albums that don't include all the songs (Not sure why since it is all from the same artist). They don't have every artist, though I don't think any of them do. You cannot redownload a song you purchased but lost without having to pay for it again.
I don't know. If your son is really into music a subscription service might be a better way to go. And from the looks of it there are other single download sites that cater to your player so unless you really want iTunes (And are fine with converting everything minus podcasts) I would go with one of the other services listed in that link.