If he used the TurboTax online editions you will have to know the User ID and password for the TurboTax online account. If you do not have this information, contact TurboTax support and they can tell you how to proceed and which documents are needed to prove your identity and marital status.
Support is open from 5am to 9pm Pacific time, Daily
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
You can also go to the IRS to get transcripts or an actual copy of the tax returns.
You can get a free transcript from the IRS - https://www.irs.gov/individuals/get-transcript
For a fee of $43 you can get a complete federal tax return from the IRS by completing Form 4506 - http://www.irs.gov/pub/irs-pdf/f4506.pdf