Kensuke tells Michael to keep everything a secret until ten years have passed, when Kensuke passes away. In the postscript, it is revealed that 4 years after the book was published, Kensuke's son in the novel, Michaya, had survived and wrote to Michael asking to talk about Kensuke. Michael notes that he laughs just like his father. The book does not tell us whether Kensuke died, but judging Kensuke's age, and the way Michaya asks about his father, Kensuke, we can probably guesstimate that Kensuke is probably dead.
Hope this helps xox :)