Son died in september 2016, i need to file a tax return for him, he used TurboTax, but i do not have his log in information

I’m sorry for the loss of your son. You can begin a new account for your son to file his final return. If you are your son’s executor/personal representative, you can obtain a copy of his last years return from the IRS.

See the link below for the requirements to request a copy of a deceased persons return.

Deceased Persons - Getting information from the IRS