Just a shot in the dark here, but try going into iTunes, then account information. About halfway down is iTunes in the Cloud. Click manage devices. If your old phone shows up, click remove.
Only as long as he associates that SIM card with his account/phone number. When the iPhone 4 originally came out they came pre-installed win new micro SIM cards that automatically got associated with your phone number upon activation (and superseding any old SIM card numbers associated with your account).