I have been reading all the posts I can find on the subject as we too are having sleep problems again. My son is 20 months. He will go to sleep in his room, but wakes up a few hours later screaming, the only thing that will help him go back to sleep is to sleep with us.
I have spoken to his pediatrician and she tells us it is just a stage. She claims the only way to get him past it is to let him scream. I am not opposed to letting them cry it out, but do not have the heart to let him scream for hours on end. So we just let him sleep with us. Personally, I think it is a seperation anxiety issue and for whatever reason, right now he needs that closeness.
Keep a good bedtime routine. Add a nightlight if you haven't already, and we also have added a teddy bear that plays soothing nighttime music (actually, I think that suggestion came from another post of Diva's). I have also tried just sitting in a chair in his room, and often gently rub his back. He knows I am there, and it seems to make him feel safer. So far these things have improved the situation, but have not helped him totally overcome it. Personally, I think they will just grow out of it.
Good luck to you.
He could be having night terrors. My DD did that for a long time. We just let her sleep with us until she calmed down. Then we reintroduced her to her bedroom again. Also, does he have a favorite stuffed animal that he can sleep with? I would also try playing some music while he sleeps or a peaceful video.
Does he sleep during the day how long and what time is his nap ,what time is it he wakes up, in the middle of the night ? another thought, has he had anything new happen recently ,just before this started ?