It means that the fellow in question (Gatsby) is like David Belasco, a theater producer and playwright who is known for his realistic sets.
So, he compares Gatsby to David Belasco.
Explanation:
In the Great Gatsby, a dude who was at one of Gatsby's many parties who was expecting Gatsby's books to be fake just like everything else about Gatsby is fake. But the reverse was the case - the books turned out to be real. But Gatsby has never read them. At this point, the guy called Gatsby, "...a regular Belasco". Actually comparing Gatsby to David Belasco, a stage/theater producer and playwright.
David Belasco is known to have pioneered many innovative forms of stage lighting and special effects which he used to create realism and naturalism.
