Sorry if this was already proposed....since I'm not sure I want to read everyone's theories yet in case there are real spoilers, but my own is that this will have echos of Flowers of Evil.
Theory: There is a real son that got sent away or placed in hiding because the parents found out he's a psychopath or sociopath, and they brought in the ML as a substitute. The real son is the mastermind/puppet master behind the guy with the heterochromia eye condition and wanting to take down his replacement (who is probably the illegitimate son of someone powerful, because, why not? lol). At first I was suspicious of the 2ML, but now I think the boy he played with as a child was the real son, and he's going to think the ML is that kid who did whatever horrible thing he witnessed. (This could also set us up for some tantalizing trust-distrust push and pull tension between the OTP as well as some love triangle jealousy from the ML.)
Another additional possibility is that the real son was also the one that also caused whatever accident killed the newspaper chairman's son, and injured the FL when they were kids. Since this is a K-drama, the dead son is probably also not actually dead and is probably the 2ML investigating what happened all those years ago. 🤣
I noticed during my rewatch of ep 3 that the 2ML said that he likes seafood when asked -- didn't the mom say her (real) son liked seafood to the ML when he was a kid and shoved a piece of sashimi in his mouth? So another theory is that the 2ML is maybe the psychopath/real son (he is a psychiatrist and would know how to act normal?)...when he is telling the orphanage story, he is actually talking about himself and the scary thing that happened was he killed the playmate from the orphanage, which was what got sent him away....the playmate could also have been the dead Hong son, who was killed because he was a witness to whatever terrible thing the real Paik son did. Regardless, I wonder if the cold cases he is investigating is related to whoever is the psychopath son (since this is a k-drama, one or both of the sons must be alive and one of them must be unhinged 😅😬.)