You'll probably have to do a reset to factory settings. It's not quite as simple as logging in and out, the account you specify when you start the phone for the first time becomes the "owner account". So, wipe the phone and do over.
There might be another way, but this is the impression I got of the whole first-run process.
SeeK is correct. Your gmail account (the one you setup the phone with) is linked to the phone. He will need to setup a gmail account. It can be a dummy account just to set the phone up but he will need one.