Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
Hi! Sorry to hear about this situation. He should first sign in to account.microsoft.com/services and check whether he currently has a Microsoft 365 subscription that is active or has a past-due balance. If so, he will not be able to join the shared subscription yet. He would need to either cancel the subscription, or if there is an outstanding balance, pay it first and then cancel it before he can join the shared plan.