By the way he talks. Euijae has a habit to talk softer to weak/younger/elder people. During the time Sayoung knew J, he couldn't talk or see at all due to his body condition (was exposed by poison when J saved him) that made him bandaged like a mummy, and it's always J who talked to him at that time.
Well thinking how long Euijae disappeared and what kind of tone he used when talking to young Sayoung, no wonder it's a bit hard to immediately remember. Even Sayoung only recognized that Euijae is J when he talked the way J used to do to him in the past.
+ with what happened to Sayoung after J's disappearance, his bad memories probably kinda covered the memory about J.

Guys, if you are hoping for any romantic scene like kissing, confession or even R18 things from the current timeline soon....... there's none.
The story will get heavier especially when Euijae knowing the truth about previous timelines and how sad it is for previous Sayoung to lost Euijae again and again.
Also, when Euijae knows that Sayoung was the kid he saved and got close with before he went to the rift, he was so incredibly happy that his usual attitude immediately changed to a sweet one, especially after he knew what happened to Sayoung after J's disappearance.
There will be also a time far from the current chapter when Sayoung lost consciousness for weeks and public was getting restless without the strongest hunter showing up, so Euijae decided to show up as J to reassure everyone, and he was successfully shocking everyone.
And the apocalypse thing will start to happen as well.