He did partially forgot his memories
Spoiler……………..he did but then he regained them later but decided to keep that to himself
I don't blame him one but either
*bit
He didnt keep it secret because he wanted revenge. He did it so Hyung wont look anywhere else but him. They both are paranoid lol. They are scared of loosing each others even though neither of them would lol.
Did he actually lose his memories?.... hmmmm