Access to iCloud Account when the iPhone is completely broken

My son has totally broken his iPhone. We are not sure if he has fully backed it up.

We need to access his Apple ID to a) Increase his storage subscription, b) if there's any life in his phone at all back it up using Apple Devices, c) transfer his backup to another iPhone.

However we cannot access his iCloud account because in order to log in to his Apple ID there's a two step verification process which we can't use.

What can we do?