I’d rework the intro, maybe paint the scene a little rather than just say “discovered”.

The “Aw Hell” remark isn’t enough when the perch is taken. I’d add a touch of mythology (simile) as the narrator describes the bass. I understand your market limits literary prose but the first time I saw a ten pound Black rising, I peed in my pants. I need more reaction, personalization, especially from a child.

“Jim Slade discovered the giant bass he called Hookmouth late one
evening in July”

It rose up from the depths, a huge, undulating shadow. Opening its enormous mouth, the bass sucked in the six-inch bluegill, then turned and dove for the dark, cool bottom of the pond. Jim’s line snapped and he thought for only a moment that the huge bass had looked him square in the eyes as the perch disappeared. Jim named him ‘Hookmouth’ right then and there.

“At thirteen, Jim was small for his age, but his dad declared that Jim had a good head on his shoulders, and so should be allowed to ride his bike to Uncle Al’s in order to fish in the pond there.”

A little awkward- “the pond there”

At thirteen, Jim Slade barely came up to the other boy’s chins but his dad claimed he had a good head on his shoulders and should be allowed to ride his bike to Uncle Al’s pond.

The character is likable, the voice subtle but you add a ton of tension to this.

Dean

The structure and balance of the story. It is clear early on what the story is about, and there is a patiently built tension, that culminates in the final paragraph. (Even though the first paragraph states the outcome).

There are memorable word choices used (”… his offering …”, “… dredged them in beaten egg …”, “”… reproduced obligingly …”)

I would change …

The second use of the phrase ” … a good head on his shoulders …” so soon after the first use.

When do we get the rest of the story ? !

A small suggestion…you might consider a transition sentence or even a sub-heading between the paragraph beginning “There ain’t nothin’ better for supper than” and the next paragraph where Jim is on the bank fishing.