If he dies before the election (or swearing in), the DNC chair, Tom Perez (a Hillary plant)
DNC House and Senate leaders (Pelosi and Chuck U Schumer)
Democrat Governors Association Chair (Phil Murphy of NJ)
Would select Presidential candidate.
https://www.vox.com/2020/3/27/21186614/what-happens-presidential-nominee-coronavirus
Filling a Vacancy on the National Ticket: In the event of death, resignation or disability of a nominee of the Party for President or Vice President after the adjournment of the National Convention, the National Chairperson of the Democratic National Committee shall confer with the Democratic leadership of the United States Congress and the Democratic Governors Association and shall report to the Democratic National Committee, which is authorized to fill the vacancy or vacancies