S.C.
My son is about the same age and is typically a great sleeper as well. Lately, however, we've had difficulty getting him to bed - he'll fall asleep in our arms and then will wake up as soon as we put him down in his crib.
One more possibility to add to the mix is bedtime separation anxiety. Kids start to experience it right around this time and although we tend to think of it as a day time issue, it is just as concerning to a small child when left alone in a dark room. If you've ruled out a medical issue, try sticking to a bed time routine that starts and hour or two before he usually goes to bed. It won't eliminate his fears entirely, but it can help.
Best wishes.