I don't know... Why don't you ask him?
I ran into my ex-boss, he looked interested but was too nervous to make a move. On fb, we were flirting until this year when I sent him a pm for happy b-day. Not only did he not respond, but he hasn't liked any of my posts since then and for 7 months. Why?
Does he have a wife or girlfriend? It sounds like someone may have encouraged him to stop talking to you...maybe some of his messages had been found out.