apple id cant log in or find email details plus does not know what anwsers he put

ive had my iphone 5 for two years and just got an upgrade so we gave the phone to our son whos 10 i set the phone back to factory which wipe everything off.we told him to wait but he set up a icloud account with email then went to change the sim to another network which its still locked to my o2 account when it did not work he changed the sim back but forgot all email and icloud details now we cant get into the phone help please....