After 2016, I doubt there's any way to convince him that he can't win. Remember, Hillary was leading by a similar margin at this time.
But Hillary Clinton was arguably an even more flawed and hated candidate than Biden. The only people who truly hate Joe Biden are on the far left, and even they hate Trump even more. I mean, do you know anyone who would crawl over broken glass to ensure that Joe Biden never sees the Oval Office? I don't.
The only way I could see Trump dropping out as if he saw it as a way to preserve his dignity and image—and that would require him to frame the 2020 election in a way that would make it appear as if it were rigged against him. Vote by mail might give him that window.
Actually, let me put it this way.
If Biden is elected, he'll get a year or so of basic criminal lucre out of the office, sign a few wish list Dem Bills (if they keep enough of the House and get the Senate), and be outta there.
Whoever is next in line will be who really gets elected. He's a placeholder.
"You have to pass it to find out what's in it. " would apply to the White House, too.
There is no one on the Left I would want to see in that office again in my lifetime.
Ironically, I who did not want to elect him, do not want him to lose this time. If he drops out, who will replace him?
Who that can beat Biden and keep exposing the crony corruption in DC?