去DMV過戶好了, title ！= registration

Go with your nephew to DMV.  Ask for transfer “Title Only", not registration/insurance.  A car need insurance only when it is registered.  But you are transfer title only.   Some state allows it. 

Fill out the title transfer form using your nephew's US address and his mother's name.  DMV will mail the title (under her name) to his address.

Once your nephew got the new title, have his mother sign it over to you.  You take the title to DMV 

Above is just my best guess, individual state procedure might differ.