I think he knows everything. He said somewhere in the beginning when Mieum was sleeping that "I missed you Mieum" and hugged him gently (And Hajoon said that "Does he still love him?" a few Chapters ago). Also he recognized the scar on Mieum's neck (it is probably from the time when Mieum protected him and got hit and lost his memory... Based on those short scenes that he remembered from that time) and when Jitae had his nightmares and they were talking, he said he knows what kind of person Mieum is and that "this time" he should run more far.
And also when Mieum asked him if they have met before, Jitae got excited and tried to help him remember T_T not to mention Jitae is rich with connections, he probably already did a background check on Mieum (I think they have also talked about it already, when they made the contract in the car).
So the one who needs to remember is Mieum, but probably when he starts tk remember he will blame himself for everything that happened to Jitae T_T
The dots are connecting friend
I'm excited to see what Mieum will do. I hope their backstory gets a couple chapters once its revealed (⊙…⊙ )
This recent chapter got me giggling lmfaooo but I wonder what's gonna happen when young master finds out about their connection