Is there some reason why you don't want to simply use a wiki?
My understanding is that you don't want the public to be able to change things - just your client.
No problem. Set up one or more user IDs for the client's use, and require users to be logged-in to make changes. Take the login section off of the home page, and give your client a private log-in page. (So as to not confuse users.)
Some reworking of the UI and configuration options may be needed to avoid confusing users. Obviously, it shouldn't say "wiki" anywhere, because - from the public's standpoint - it isn't a wiki.
Of course, this depends on what you are trying to accomplish. If it's simply to have a site that is easy for your client to update himself, perhaps a CMS is the solution. On the other hand, perhaps your client has used wikis, is comfortable with the concept, and has said "I want that". Give him that, then.
I think a wiki is a LOT easier to learn than a CMS. If a wiki will do what your client wants, I think that might be the best choice.