Very sorry for your loss. In order to find a return your nephew filed using an online account you would need the account and user ID he used. Or if he used desktop software, the return is only on the hard drive of the computer he used.
You can get a free transcript from the IRS or for a fee of $30 an actual copy of your tax return.
https://www.irs.gov/individuals/get-transcript
https://www.irs.gov/pub/irs-pdf/f4506.pdf