en_tn_condensed/pro/04/03.md

When I was a son of my father

This refers to the time when the writer was a child still living under his father's care. AT: "When I was still a boy learning from my father" (See: rc://en/ta/man/translate/figs-explicit)

the tender and only child

Here the word "tender" refers to a young age at which the child is still weak. It forms a hendiadys with the word "only." AT: "the tender only child" (See: rc://en/ta/man/translate/figs-hendiadys)

Let your heart hold fast to my words

Here the word "heart" represents the person's mind. The writer speaks of remembering words as if the heart were holding on tightly to them. AT: "Always remember what I am teaching you" (See: rc://en/ta/man/translate/figs-metonymy and rc://en/ta/man/translate/figs-metaphor)