"Setting up email address on new phone.: Trying to set up my son’s phone and it has a gmail address assigned to it. He doesn’t have access to the assigned email address and nor does it exist when I look it up. When I try to add his iCloud email it doesn’t accept it and all of the settings are greyed out until I confirm the email address, but again I can’t. Thoughts?"
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Getting Into your Device:
A. You Need to get the Correct eMail Address in there. This email address is his Apple ID --the email address he uses for logging into all of Apple. So, it was logged in, at sometime, by some one. Is this a used device? Has someone had their hands on this device? If this is a used device, then this is the site the seller should have used, prior to selling it to you, including the iCloud logout: What to do Before you Sell, Give Away, or Trade in your iPhone or iPad - Apple Support
B. Making a Store Appointment:
So, if this has always been been his device, then Apple out to be able to change it for you. You'd et up a Genius Bar Appointment at your local Apple Store. There, it will be diagnosed, and you'll be provided with your options. IMPORTANT: Be certain to bring along a proof of purchase (i.e. sale receipt or original packaging) and the iPhone, itself.