Sprint iPhones are CDMA and AT&T iPhones are GSM. The two systems are not compatible, even if the phone was unlocked. I'm afraid that your son will need another iPhone if he wants to use it with AT&T.
Sprint iPhones are CDMA and AT&T iPhones are GSM. The two systems are not compatible, even if the phone was unlocked. I'm afraid that your son will need another iPhone if he wants to use it with AT&T.
while the CDMA and GSM are combined in one model, if the phone is locked to Sprint, it will not work with any other carrier, including AT&T. If you can' together the phone unlocked, you will need an unlocked or AT&T phone.
while the CDMA and GSM are combined in one model, if the phone is locked to Sprint, it will not work with any other carrier, including AT&T. If you can' together the phone unlocked, you will need an unlocked or AT&T phone.
I've found some more info about your situation that makes it unlikely that you will ever be able to use the phone on the AT&T network. While the hardware is identical between the GSM and CDMA versions of the iPhone 4S, the modem firmware us different between the two versions and can't be changed.