Well, if he can't manage on his salary w/o your $1800, he's not going to be able to afford that apt long. It's good that you never re-signed the lease, you just need to tell the landlord that you will not longer be living there or financially responsible for the place (get a quick consult with an atty on how to do this) and then leave X to his misery.
I don't know that I'd tell him until I showed up and removed my belongings. Why give him a chance for mischief--or an opportunity to sell your things? Get your stuff out asap and move on.