Yes he does.
he doesnt know yet
At the end of the first chapter Dowan says how Chamin won't remember it the next day. Then the beginning of the second chapter has the teacher talking about sexsomnia.
Dowan knows about sexsomniael but doesn't know that Chamin has it. The first chapter is ahead of the timeline of the rest of the chapters. After the first chapter, they're now showing how the two of them started doing it, so I don't think Dowan knows right now that Chamin doesn't know they're doing it, but I'm guessing he'll get to know soon
But that's later. Right now we're still in the flashback where Dohwan is not aware of it yet.
HE DOESN'T KNOW THAT CHAMIN DOESN'T KNOW(￣∇￣")