Hi benm220,
We understand that your son isn't able to sign into Apple Music, and we'd love to help out.
To participate in Family Sharing, all family members need to have their own Apple ID account. While children under 13 can't create an Apple ID on their own, the family organizer can create an Apple ID on their child's behalf. So, if you've already created an account for your son, they should be able to use Apple Music.
To help narrow this down further, can you let us know which device you're trying to sign into Apple Music on? If you try to sign in on another device, do you run into the same error?
Are you able to sign into other services with that Apple ID account? For example, can you sign into the iTunes Store and App Store without an issue?
If you see a similar error when trying to the iTunes Store and App Store as well, the steps on this page may help:
If you can't connect to the App Store, iTunes Store, or other Apple services
Take care!