Sleep changes

My two year old has generally been a good sleeper, sleeping through the night and putting himself back to sleep without help if he wakes up. Lately he started having trouble though, and either has an early wakeup of 4:30 or 5:30, and won't go back to sleep, or a night wakeup and needs us to go lay down with him. I feel like we've tried everything, we think he's on the right schedule because he is tired when he goes to bed & falls asleep quickly. He doesn't always nap anymore but it seems to happen regardless of nap. I wondered if he was starting to get nightmares or get scared of being alone in his room or something. Or that he was getting cold, or because he had a cough, or his nightlight was too bright. But trying to change those things hasn't helped. Anyone gone through something similar and figured out how to get them back to sleeping well??