It's the game some guys or even girls play on other people who are already in a relationship.
Step one. Get it into their mind.
Step two. Wait and see.
He purposely mentioned it to you and if you continue be nice to him or befriend with him. He may see it as a crack to slide in, any vulnerable time. If you really don't want it than you must draw the red line.
Completely cut off. In my experience around, this is how things eventually develop and progress. You should not care, how he feels to be honest