I tend to agree with Deborah, because if this all started suddenly, I'd look at whether he needs more of your time, or if he's feeling upset about something. Does he sleep during the day? Perhaps he's being put to bed too early now that he's growing up a bit? Has he become afraid of the dark?
Stay as calm as possible, and, as Woody says, read to him, make sure he's not hungry or thirsty, and then tell him that you're going downstairs for a while. Kiss him goodnight, and leave, with a smile on your face. If everything else's well with him, he'll soon get into a routine.