Apple ID locked on kids phone

My nephew's phone was under a parent's Apple ID. We got him his own Apple ID and did not add him back into the family group. He's 15 and wanted to be separate. Now when he goes to update his apps the iPhone asks him to sign in and won't accept his Apple ID. It keeps asking for the parental ID even though he is no longer part of that family group and has his own separate id. It wants to unlock his iphone using the parental phone number.

I thought maybe it wanted the parental id because his apps were originally downloaded under that ID. So we had the parent add him back into the family group but it is still asking for the parental Apple ID in order to update his phone.

Can anyone tell me what has gone wrong and how to fix it?

Thank you for any help out there!