Z.B.
It is completely normal for his sleep habits to change. He may have gone to sleep on his own easily in the past, but he's not doing it now. This is actually very normal, as his awareness of the world has really changed. Now he knows he might be missing out on something if he goes to sleep, and he doesn't want to miss out. Or it could be that his imagination has really grown and he is truly scared of shadows and what might be under his bed or in the closet or whatever. Again, totally normal.
Why not just stay with him until he falls asleep? It will help him to feel safe and calm. And once he feels safe and calm and can actually fall asleep without a fight, you'll be able to start leaving the room sooner. It won't last forever. It's kind of a pain right now, but it won't last forever.
There's no need to make leave hi to cry. Just help hi through this rough patch. You won't regret it.