This guy also sounds like Ji Chang Wook when he talks as well as the facial expression. I am confused since there is nothing online about it nor is there a casting for this guy

It’s neither.  It looks like Lee Jung Woo who is listed as unknown. But I can’t be sure. What i’m sure is that that actor is neither of the two you mentioned. 

It is not Lee he joon . Check the soompi article about the cameos. If he were in episode 5 as you say, he’d be listed and everyone would be gushing about him. 

And after a search, it’s https://asianwiki.com/Jang_Young-Hyeon_(1993)

Do a google dearch, you’ll recognize Lee him. 

Thanks I can finally feel assured and not confused