Can I factory reset my Sprint iphone 5 so that my son in England can use it on a GSM network? I have the unlock code from Sprint

I have got an IPhone 5 on the sprint network. I got a new phone and got an unlock code from Sprint. I want to be able to give my IPhone to my son who lives in England. Sprint said that they work on CDMA and not GSM like most other networks so I wouldn't be able to give it to him to use. Is there anyway to do a factory reset so that the phone can be used in England?

Thanks.