As far as I know, if his iTunes account is not associated with his phone, you would have to do a master reset to change that.
You can either do this from iTunes when it is connected to a computer or I think under the "General" tab in the settings menu. Make sure you are setting it up as a new phone as I suspect setting it up from a back-up will result with the same iTunes acct being used.
If you have any info on this phone which you would like to keep, you would have to save it as performing this restore will erase the phone completely.
Also keep in mind, any apps purchased with YOUR iTunes account will no longer be available to this iPhone. Only apps purchased thru HIS iTunes account.
During the setup procedure, it will ask for the iTunes account for the phone.