If the home connection is a 56k modem, set Internet Explorer's settings to 'Dial only when a network connection is not present' on the Connections tab (Tools > Options).

If the home connection is broadband, then the modem/router is probably being seen as a network connection and, therefore, that complicates things. The laptop will then have two network connections - the wireless network card for the office and the USB broadband network adapter at home. So you can't set Internet Explorer to work out which one to use every time.

Perhaps you could use a batch file / VB script or something like that to disable the WLAN card when he is at home and then another to enable it again before he shuts down and goes back to the office.

Alternatively, use hardware profiles - this is exactly the kind of thing that they are for. You can set a 'Home' hardware profile that has the WLAN adaptor disabled and then boot to an 'Office' hardware profile with the WLAN enabled and the USB broadband disabled.

Let us know if that helps