Then they probably haven’t yet changed his email address. As soon as they change it, people sending him mail will get bounced....unless they configure their filters to ‘eat’ messages sent to his old address, which is very unlikely.
For a higher up in a school like he is, changing an address isn’t a trivial thing. That would probably be a last resort. If it gets too bad, they will probably grant permissions, and add his box to his secretary’s mail client, having her sort and try to screen his mail for him until it blows over.