Our son has somehow changed the email address on his apple id without changing the apple id

Our son has somehow changed the email address on his apple id without changing the apple id and can now no longer get into his apple ID on his Ipad. Can you please help

His Apple ID is ****1@gmail.com but he now has the email ****2@gmail.com in the Ipad ID only. he doesnt know the password for this so we are locked out of his apple ID on the Ipad only. Itunes is fine as we reset the password

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