Hello there,
We see you have been experiencing some account access difficulties and we want to help you out with this the best we can.
At Meta Quest Support this piece of account information is one we always need to request, primarily for account security but also for troubleshooting account related issues that you may be facing.
As you mentioned that your son is an existing Quest user with purchases on his account, the good news is that you should have received emails from Meta Quest Support before. Please check all inboxes and junk folders of all possible email addresses that your son may have used.
We really hope you manage to get to the bottom of this.