The focus would be is that a young man going through puberty has a fascination with breast milk, one way or another, he gets a taste, either directly or from a bottle, and it has effects on him.
I haven't read any stories with breast milk, but I have read a couple where the kid's sperm has an effect on the women around him (so he goes nuts shooting his sperm into everyone's foot, like a real wack-a-doodle). Can anyone say "American Pie"?
i did read a story where it mentioned that we can't access 100% of what our genetics could offer to us because of different reasons.
That old canard gets dug up repeatedly, but it's all based on bogus quasi-science. All they've ever noticed is the brain never lights up completely, and someone guessed (with no basis) that human's only use 10% of their brain. But have you ever tried to activate every single electrical circuit in your home computer. It simply can't be done, and if you tried, you'd fry the entire thing. The idea that some 'genius' might someday use 100% is pure fantasy.
Add to that a recent report questioning the use of fMRI (functional Magnetic Resonance Imaging), which finally determined all the 'miracle' observations using it were the result of a statistical fluke in the software (one researcher, to prove the point, tested a dead cod's response to spoken English words describing relationships, and proved dead fish respond better to certain phrases (it's brain lit up with certain words). Except it's a damn fish, and it's DEAD! So the next time you read a report claiming something unbelievable (like having people you never see pray for you), it literally is unbelievable. It can be corrected with error correction software added to the machine.
As for your idea, it's not really a complete story idea yet. You have a mechanism, but no real conflict. Does drinking one small juice glass make you grow 8" overnight, or do you need repeated glasses to grow a quarter inch at a time. And where's the conflict in that? In most cases, most stories either rely on mine-control or psychic abilities, and base the conflict on the bad decisions the people gaining the abilities make.
A good story would be if a young boy got smarter (for very short periods), after stealing his older sister's breast milk. He could then try to steal it from his cousin in order to get better grades, only be get caught by his sister, but you still have no kink. For that, the sister would have to bend to his will and supply the milk, for whatever reason (and again we're back to mind-control magic).
The key with this type of story is to identify what conditions might cause a sister to voluntarily feed her younger brother her breast milk, instead of using it to feed her baby?