iMessage set up for child on family sharing

I have just set up my son on an old iPod touch and used family sharing to give him an apple ID. I am trying to start up his iMessage and FaceTime account.

I go to the screen it asks for apple id. I put his in and his password and it asks me to verify, then nothing happens and we end up back at the beginning of the sign in.

If I use my apple ID then he has messages via my iMessage, which isn't really what we want.

I can't figure it out?!