H.L.
My so did this exact same thing at the same age. It was sad/frustrating for me too. I asked alot of people and noone knew what to say about it. Darwin would even wake up in the middle of the night (sometimes 3 or 4 times) with the crying.
I guess it was a phase, because as quick as it started, it stopped. One thing I did do was, my parents sent him a really soft cuddly teddy bear that we named Oso, When I would tuck him im I would also tuck in Oso and talk to Oso and tell him "not to be scared because Darwin was right here to take care of him" and things like that.
It seemed to help Darwin by giving him responsibility over helping Oso not to be scared.
It is over for us, he sleeps and wakes up happy again.