I took his key but I can't find his room anywhere lol

thanks.

If you took it off him after getting to him on the balcony, you don't need it. It's just for that door that the security guard stands outside. He doesn't appear to have any rooms other than that. Mr. Burke takes up the other room on that floor, along with yourself - if you have the suite - and the Chengs.

Then where does he sleep? : /

In his bed.

Go through the door the security guard sits outside, then cross into the little "garden" area and go in the other red door which is on the left-hand side. See the maize of rooms? That's his quarters.