I could be wrong but I thought the account could only be associated with 1 email address. Part of the reason is to stop players from transferring or selling resources (saved game data) to other players.
Of course, there are valid circumstances where a change of personal email address is necessary but I guess this could only be done by the helps from an advisor once he or she satisfies with the provided supportive evident that the case is legit.
For your situation, I think the best option is to make sure your nephew does not delete or reset the game and start a new one with that account as each account has its own player's ID and part of the game save progress is synced to the server. Take note of that player ID and contact EA Help Center (from the link below) and get further assistance.
Alternatively, you can re-install the game on your device and recover the progress with your nephew's email address and get your nephew to register a new installation with a DIFFERENT or a new email address or else he will reset the game on your device.
