And quite possibly tragic. The odds are strong that if his family has been denying that connection though it is that recent, it was almost surely related to a wartime rape or prostitution situation, even possibly more than one. Korea was occupied by Japanese troops during WW2, and by quite a number of Chinese troops during the Korean War.
I've been a librarian for nearly 30 years. I've seen a whole lot of sadness and family strife related to genealogy research. Most of the databases out there are rife with errors. My own family is quite close-knit and open about even our family shadows, and I can think of at least 20 errors in various databases that have been put out there by people who have decided that they are related to us. There is one lady I know of who is very stubbornly insisting that her grandfather was one of my great-uncles, which I happen to know is impossible because he died at age 3, and is buried next to my grandmother. What I'm saying is, please don't take this hobby too seriously.