I am very sorry for the loss of your son.
To file a tax return for him you would need to set up an Online account for your son, you can do that by visiting the following TurboTax website https://turbotax.intuit.com/.
When you reach the Personal Info section of the interview you will see a question that says" I'm preparing this return for( )who passed away." you would then select"Yes" and enter the date of his passing. You can then continue through the tax interview and TurboTax will walk you through each section step by step.
Here is an FAQ with additional information:
https://ttlc.intuit.com/replies/3300219