My guy to summarise what everyone else has sai: MOVE ON!

Case closed.

Don't give him if you guys break up he can lock you out of your account & change you password, if he insists ask him for the PIN of his ATM card in return to show you trust each other and have nothing to hide 

Besides which he may have multiple email addresses so giving you the password to the innocous one has no meaning

Nigerian Men cheat because they can! they have willing accomplices everywhere so don't blame them for infidelity, just like prostitutes work because they have steady clients if they discover the business is not lucrative they will look for something elase to do.

I had a fling with a girl once (I am married by the way) and as part of our discussion she was telling of how her fiance was unfaithful to her impreganted someone else and she was really bitter about it. Meanwhile this discussion took place while we had just spent the night together and our relationship was ongoing while she and her fiance were engaged.

So please lets not judge the men.