I have not personally dealt with a situation like this, but my girlfriend did with her son. When he was about 3 and a half, she explained to him that he was old enough now to fall asleep on his own. She told him the plan that she would sit on his bed for 2 nights until he fell asleep, then stand in his doorway for 2 nights until he fell asleep, and then sit at the top of the stairs un til he fell asleep. He understood and within a week he was falling asleep on his own, and she hasn't had any problems since.
Just one idea!