Change HomePod mini account remotely to family member

I am overseas and my son has HomePod mini at home which I can view remotely on my iPad.

The HomePod is logged into my account (primary user) which means there is no filtering to what he listens to (he is 12).

I have set up family sharing and got him iCloud account but how do I change his HomePod remotely to make it work with his account and not mine?

On a second point when viewing the HomePod remotely using the Home App I can see when the HomePod is playing but when I click to see what he is listening to all the details are blanked out (including controls such as volume etc.). Any way to fix this?

Thanks