This is terrible, and a huge loss to the lunar development community. I just saw him in January at the lunar landing science workshop at Ames. He had finally come around to oppose SLS. Condolences to his family and other friends, RIP, and ad astra.
[Update a few minutes later]
More from Leonard David, who was as shocked by the news as I am. I hadn’t been aware that he had lung cancer.
[Update on June 8, 2021]
Paul’s widow, Anne, asked me to update this post to note that, contra a comment here, Paul had quit smoking in 1988, and was informed by his doctors that it was not the cause of his cancer.