You're a placeholder, pure and simple. He won't change as this is actually the exact scenario he wanted from the start.
You're obviously an attractive enough prospect to keep around for his convenience (sex on tap and presumably you facilitate other things in his life), and you have also proven to be suitably docile to not demand too much, so he gets a decent return for minimal effort. But he isn't into YOU. If he was, you wouldn't be at the same point you started at after 18 months.
He has essentially told you by his actions (or lack of them) you're not in a relationship with him, and has taken the fact you haven't kicked him to the curb as your tacit approval of the current situation.
My main bugbear in relationships used to be people lying by omission and game playing like this. Such a waste of time and effort. It's only solved by putting yourself first in the pecking order and walking away from it.
When he finds a better prospect, you won't see him for dust. Hopefully, you won't have wasted too much of your life on this twatbadger by this point.
Think about what you actually want in life - I doubt he's actually providing any of your real romantic hopes and dreams. And by convincing yourself this is ok, you'll never get anything more until he chooses to bugger off.