This is where things get interesting.
Legally, he doesn't owe them a single cent beyond what they were paid. They agreed to do a job for a specific amount of money, and they were paid exactly that. The fact that the movie later became a massive success doesn't change the contract.
Nobody, not the actors, not the crew, not the studio knew it would become a hit. If the movie had flopped, nobody would be offering to refund part of their salary.
That said, there's a difference between what's legally required and what's morally right.
He can absolutely afford to share some of the upside. A few million dollars would be a tiny fraction of the film's total earnings, yet it would transform the lives of the people who helped make that success possible.
More importantly, the goodwill and respect he would earn from doing so would be worth far more than the money itself.
Obsession's director has responded after the art director said she earned less than $7,000, while the movie made nearly $300 million
He said the crew "deserve to be recognized" but that those who take financial risks are rewarded the most