The blackberry device is secure because it works with the various id's and codes on the phone via the server. Email and calls are directed by the various services to the SIM in the phone. If he lost the SIM a new one would be required by the carrier to receive calls. Attempting to do a call forward requires the original phone or a call to ATT.
You can provision a phone to be a spare but it will still require someone to switch email and other data to the new phone.
When a message is delivered to a BlackBerry Smartphone, the Blackberry Enterprise Server will deliver it to the BlackBerry Infrastructure. The BlackBerry Infrastructure will check if the BlackBerry Smartphone is correctly provisioned, this includes having the correct SIM and IMSI assigned. If the correct SIM / IMSI is not in the BlackBerry smartphone, then the BlackBerry Infrastructure will fail the message back to the BlackBerry Enterprise Server.
Have you tried to glue the phone to the boss?