Best thing to do is complain to your son & get him to fix it... haha...
1st thing: go to Settings --> Music & if anyone (you or your son,) are signed into Home Sharing, & turn it off.
(Home Sharing lets you listen to songs/music/movies/podcasts that you have saved on iTunes or your computer & lets you listen to it on your iPod without the need to save the media on your iPod/& play through home stereo etc..)
or (most likely) ...iCloud is copying his songs/movies/downloads to your iPod's iCloud...
Just go to Settings --> iTunes & App Store, & turn off Automatic Downloads.
...also, to delete, just go to the Music app & whatever you want to delete, just click the " ... " 3 red dots on the right of the song/album/artist...
& choose Delete From My Music
(it MAY/MAY NOT ask if you want to remove the file &/or just delete from your iPod (i forget exactly,) just choose to delete/remove from the iPod, but not delete the media.
...there's a few ways to copy/share/download between devices, so it can be confusing & tricky 🙂