What are his day sleeps like?
My DS is 17 months and we cut him down to one day sleep about a month ago as he was waking at about 5-530 and it seems to have fixed things up.
I usually put him down after an early lunch (1130) and I don't let him sleep after 4pm, so even if he needs another sleep later I'll make sure he's awake by then.
Now he'll usually only have bad night if he's sick or his teeth are playing up (his top and bottom fangs - don't know their real names- are coming through)
Now that I'm sorting his sleep out my DDs have decided to go back to waking 2-3 times each night :-( can't win..