not sure if the procedure is the same on ncl as carnival, but for us, if we as a crewmember have family/friends cruising, there's a permission form filled in and signed by the staff captain and chief security officer, so that they know why we are in a guest cabin. it's usually straightforward, but make sure it's all filled in by him before you go, as there could be problems otherwise.
Anyway - yes, do tell him you plan on coming or you may be disappointed in what you find out. It would be a pretty miserable cruise for you if you got there and found out he was engaged to someone else, right?
Just by telling him you are coming you will find out how serious he is.