The novel explained it better; the poison was melting Lsy's skin, so his face was deformed to the point that he was only alive by a miracle, because the people around him were almost completely melted.Lsy couldn't speak and his whole body was covered in bandages; when his eyes were revealed, they were opaque and whitish. The novel explained how J expected the boy to look if he was alive and looked very different from Lsy. If I remember correctly, he imagined the boy to be small, weak, and still have vision or speech problems, which would be effects of the poison.That's why he never considered the boy to be him, because he even thought the boy's personality would be slightly different from Lsy's, and that will be explained later too.

Seriously how did CEJ not notice that LSY is that boy? I mean the boy was 16 when CEJ disappeared, + 8 years = 24. In addition he's still pouty and would like to stick with CEJ 24/7, he's also super protective, does CEJ really think some random guy who doesn't know him would act like that?