At a loss here on the right thing to do...

LO is 19 months old. He's been sleeping through the night since 9 months old. Was able to put himself to sleep for night and naps since about 12 months old. For the last ~2-3 months, he needs one of us there to hold his hand to fall asleep - but still slept through. The last week, he's been up very early (3:51am today!) persistently calling for us. If we leave him alone and don't do anything, he cries for us for 30 min, sleeps for 30 min, and then repeats that cycle until ~6am when he's up for good. Brutal. If we go to him and hold his hand, he will go back to sleep - but then we're stuck sleeping on the floor by his crib.

If I knew things would magically go back to normal, I wouldn't mind going in there for a period of time. But I worry we're setting up more bad expectations and habits.

Thoughts? Advice? What did you do when your toddler's sleep went to crap at this age???