Adding Child account Home Pod

I created an apple account for my son using an iCloud email. The idea is to have him use Apple Music and be able to listen to music using our home pods. To do this I understand I must add him to the home, but when I try this I am met with the following messag in the home app: "CANNOT SEND INCITATION You can add people using an iCloud account email address with home enabled."

Home is enable on iCloud on my old iPad that has my son as user on it.

Any ideas please - thank you.