These are proper nouns.
Proper nouns are typically names or certain roles (such as family relations like "Mom", "Dad", "brother", etc.). Names are capitalized, roles are usually capitalized.
For example, "I told Mom I didn't want any" - Mom doesn't take an article, in the same manner as names like Jason, Maria, etc. wouldn't.
Using mother and son as proper nouns when you are not responding to your mother or your son is not overly common, but possible.
Mother and son feel each other well.
It's possible to use the species name of a dog or cat, or other common household pet, as a proper noun if you haven't named it yet or don't know its name.
Cat, get out of the way.
I'm going to get you, fish. (while fishing)
This is probably far more common with the "baby" version of cat (puppy) and dog (kitty).
Come here, kitty.
Aww, Puppy likes the belly rubs.