I first noticed it last month when he was playing with his little fairy dolls. He declared one to be mama and the other baby. Then mama comforted the crying baby.
Now he appoints the relationship to both live and inanimate objects and occasionally throws in the daddy. He asks/tells me, "Mama, this is mama dog and this is baby dog?" or "Mama, this is mama rock and this is baby rock... and this is daddy rock."
Not only is it cute to watch, but I think it provides him with some stability and a sense of place as I still call him "my baby" and probably will for some time.
Occasionally he moves in very close, looks in my face and softly says, "Mama, you the mama and me the baby." in almost a question tone seeking reassurance that he is mine and I am his. I always answer, "Yes, I am your mama and you are my baby."
I cherish these moments.