First, how do you know that he even used TurboTax Online to file his 2017 tax return?
Just like the IRS, even if TurboTax has his 2017 tax account, they will not give it to you without proof that you have a legal right to it such as a court order appointing you and other documentation.
It is not a simple procedure, but start by contacting customer support.
Here is a TurboTax FAQ for contacting customer support.
https://ttlc.intuit.com/questions/1899263-what-is-the-turbotax-phone-number
And why would the IRS want his 2017 tax return? If he filed 2017 then they already have it. That will not identify either him or you for a 2018 tax return. Perhaps you did not file his final tax return properly and include the necessary forms.
Suggest that you read IRS Pub 559 (Survivors, Executors, and Administrators) for a lot of good information about filing the final return and estate return and other requirements.
https://www.irs.gov/pub/irs-pdf/p559.pdf
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**