Since the response from Jerry seemed so arbitrary, I Googled & found the references listed below. I stopped looking after the first three, as each agreed with one another. Conversely, each of those references disagree with Jerry's opinion, as offered.
Verdict: "Son," in the example provided in the original question, should be capitalized as it is, indeed, used as a substitute for a name.
"Titles such as mother, father, mom, dad, grandma, grandpa are capitalized
when used in place of a name and not otherwise:
Did you hear my mom screaming?
Did you hear Mom screaming?"
"Capitalize family names like dad, mother, son and grandmother only when they are before the name of a person or when they substitute for the name of a person: He sent an e-mail message to Aunt Larson. She sent an e-mail message to Father. She sent an e-mail message to her dad. 'Would you hand me the spatula, Son?'"
"Capitalize words that are used as names. Sometimes certain words are used as names.
A) Are you going to the store Mom?
B) How are you Mom?
C) My mom is fine."