Answer:
the structure of the poem help to express the central idea because in the poem the man is teaching his son for when he needs to face the real world for example
"And the father teaching the youngster gay
How to land a fish in the sportsman’s way."
here the dad is teaching his son how to fish. But when the dad teaches him everything he could it was time
"The soul of the father is steeped in joy
,For he’s finding out, to his heart’s delight,
That his son is fit for the future fight."
Then at the end of the poem min the last stanza the father said
"A boy and his dad on a fishing–trip—Builders of life’s companionship!Oh, I envy them, as I see them there Under the sky in the open air,For out of the old, old long–agoCome the summer days that I used to know,When I learned life’s truths from my father’s lips As I shared the joy of his fishing–trips."
Then we realize that his dad did the same thing for him.