Verified answer

In this context, mommy-bear is a term of endearment, based on the special affection one has for a teddy bear. It is not a "normal" way to address a mother, but is a metaphor suggesting the mother is as special as one's teddy bear.

It is always hard to know how a writer chooses a particular uncommon phrase. I might interpret that phrase to be the son's "familiar name" for his mother. It possibly implies that the mother is moderately to thoroughly protective, since USA "common knowledge" says "Don't ever attack a baby bear because momma bear will rip you to shreds."

I would think it is from Goldilocks and the Three Bears.

There was Papa Bear, Mama bear and Baby Bear characters

It is a nickname, from teh story of The Three Bears. Do you know it? In it, there are three bears, the Daddy Bear, the baby bear and the Mommy Bear. It's a sobriquete, a nickname, like calling someone 'honey' or the like.