If he used TurboTax online, you will need the User ID and password he used to access the TurboTax online account.
You will have to contact TurboTax support for assistance with this problem.
See this TurboTax support FAQ for contacting support - https://ttlc.intuit.com/turbotax-support/en-us/help-article/account-management/turbotax-phone-number/L0Od33nMQ_US_en_US?uid=lfgviwbm
Support can also be reached by messaging them on these pages https://www.facebook.com/turbotax/ and https://twitter.com/TeamTurboTax
Or go to this IRS website for free federal tax return transcripts - https://www.irs.gov/individuals/get-transcript
For a fee of $30 you can get a complete federal tax return from the IRS by completing Form 4506 - http://www.irs.gov/pub/irs-pdf/f4506.pdf
For Turbotax, call customer support and ask for the security department. You will probably have to fax proof that you are authorized to get his tax info (such as your ID, a copy of his death certificate, and some kind of legal document saying you are his authorized representative or executor.)
Here is how to get information directly from the IRS, and what proof you need.