To be fair, Euijae doesn't know what the kid looks like or how he sounds. The readers know bc they're making really obvious, but Euijae literally only has what he said to Mingi.
Only reason why I feel like he should know is because when our ml was changed, euijae knows that the chains take away your abilities and force you to become you before you awakened, and the ml was poisoned during then, but honestly I don't know if j figured that out yet or not
I had to re-read the chapter, but I'm pretty sure the flashback he has was when he was being told about how they found LSY. iirc (minor novel spoilers but may be inaccurate since it's been a while)
LSY was found twice: once by J (from the rifts/outbreak) and the other by Jung Bin(?) and co. (from Prometheus). He was told about the 2nd time, but he obviously doesn't know that it's the same child since that happened while he was in the Great Rift. He's not quite there yet lol
J you can't be that dull, it's obvious that the kid is your poison soon-to-be boyfriend #－.－)