Unlink account from PS4

That is going to depend on what exactly you mean by logging out of his account, and possibly which account the game was activated on, and does your account have DA:I activated on it as well.  Most game systems now use a system that links games with accounts.  I know if you make the game console you are on, the Master one, you can then share games unlocked on it between accounts, but on non-master systems if you have DAI(for instance) linked ot your nephew's account, you can't then play it on your account because it isn't yours.  If you happen to have it unlocked(ie purchased) on both accounts, then you should simply log the game system out of his account and then log back into your account and it is there. 

Now if you are talking about him putting his email address inside the options for DA:I while it was on your account, then you SHOULD be able to go into DAI options, and look for the option to change it.  Now I am a PC gamer and don't own a PS4, but I thought I read that this was all handled based on your game account on the console where the game was unlocked on...but like I said, that is hearsay and not personal knowledge.