Yes he know but pretend not to know to protect her.
Okay, I really hope because he pretended not to know she misunderstood and distanced herself from him, fearing what will happen when he finds out her her real name.
I had a feeling she might be his sister since she is the princess and he is the crown prince, but I was not sure It does not come as a surprise to be that lou ming xi likes her
I have not watched a kdrama since AOS finished.
https://ladymanga.com/manga/hidden-love/