you dont indicate if you or your son has Apple Music or not...so the info below may or may not be so useful.
- the songs are obviously being added because the iPod is logged into your son's apple iCloud or iTunes account.
- there can only be one apple iCloud account running on one device at any time.
- you can turn off iTunes from automatically downloading any Music at all
- you can also turn off iTunes from downloading any Music on while using Cellular data, limiting downloads to Wifi
- in apple Music you can turn off downloading individual songs that are in playlists
difference between iTunes and Apple Music:
iTunes is an app for music purchase.
Apple Music is a monthly subscription where you can download or stream an infinite amount of music while you are subscribed but loose it all when you stop subscribing.
if your son is still using his own account for his own device(s), then the best thing to do is to decide if you want the iPods in yr possession to still be on his account, and therefore get his Music, or, log out of yr son's account on these iPods and sign in to it with your own account.
you can also consider if a Family Account works for your son and you as well.