You’d be better off changing his password and logging into his mailbox via Webmail and setting an out of office. Then he can change his password when he gets back and disable it.
If you change his password he will not be able to access his email by OWA. (True he doesn’t have access to the internet now, but this is not the way I would recommend you handle this. Why cause unnecessary stress on the user when he returns and can’t log in?) Instead you can grant yourself full access to his mailbox through the Exchange Management Console, then open his mailbox as a new profile and set the Out of Office for him. You can also set it for him through your OWA once you have full mailbox access to his mailbox. We get these requests from time to time and this is the way we handle them.
My two cents… ~ technochic