You will need to prepare Form 1310 in TurboTax. Be certain you have marked that you are preparing the return "for [your son's name], who is deceased" in Section 3 of "My Info".
Then, in the Search box in the upper right of your TurboTax screen, type "form 1310" and click the Jump To link at the top of the search results to go directly to the form.
My condolences on your loss.