1 year old sleep issues

My baby has been having issues sleeping the past few nights. He’s almost 1 and all of a sudden he will wake up as soon as you leave the room and cry. We usually put him to bed by laying next to him until he falls asleep. We don’t like CIO. I can’t figure out if it’s teething or if he might be scared of the dark or lonely or what it could be. He just got his two front top teeth in. His room is very dark as I’ve read that it helps babies sleep better. We have a white noise machine and a fan going. Is there another possibility of what it could be? He seems exhausted lately and I want him to get some good sleep and not be dealing with whatever it is.