There seems to be a lot of hint hidden in the drama about that.
1. ML’s b-day is 4/26 when he was trying to use it as a Pin number for his mom’s credit card, and old man randomly says at one time, “What day is it?” And I think someone said 24th and the old man said his son’s b-day is coming up.
2. Mentioning of Lactose intolerance
3. Both having lived in a similar neighborhood
4. Special bond the ML and old man displays throughout the game.
There are more, but I’ll stop here.
Regarding the detective guy:
I think he is alive, too. But I am thinking that the ML and the detective may pretend to join, but secretly work together to end the game for good.
I took this part out of spoiler section, per @angelight313_168’s request. I guess it’s not really a spoiler because it’s just my own theory and speculation.
“Old man’s name is Oh Il Nam (a name for 1st boy), and ML’s mom’s name is Oh Mal Soon (a name for last girl), there’s a theory that the old man is mom’s younger brother, because in the olden days, if they had a lot of girls and no boys, they would name the last girl Mal Soon or Ggeuk soon (last girl/final girl), hoping that the next one would be a boy. So when the old man was born, they were happy and named him 1st boy. But maybe the old man had to give Gihun away to his sister to raise him as her own son for some reason. (This is my own theory. )That may explain why the old man says how he used to stand/hide behind tree to watch his son play when he was little. So when he was in the Gihun’s neighborhood visiting someone he knows, maybe he was visiting ML’s mother, who could be his sister.”