Boruto might be in awe. He has always known his father as the Hokage, a powerful and respected figure. Seeing Naruto's past full of struggles, loneliness, and determination would make him realize how far his father has come. It could also make Boruto more empathetic towards Naruto and perhaps inspire him to be more like his father in some ways.
Well, Boruto could be very inspired. Naruto's past is full of struggles, from not having parents around to being constantly underestimated. When Boruto discovers this in the fanfiction, he might realize that his own problems are not as big as he thought. He could be motivated to work harder and not take for granted the things he has, seeing how much Naruto had to fight for everything.
In such a fanfiction, Naruto could share his old ninja tales with Boruto. Maybe he'd tell about his battles against the Akatsuki. Boruto would be in awe, seeing his father in a new light.
One of the most shocking revelations could be Naruto's early days as an outcast. Boruto, who has always known his father as a respected figure, would be surprised to learn that Naruto was once shunned by the village. Another could be the danger Naruto faced as a young ninja, like fighting against the Akatsuki. It's so different from Boruto's relatively more sheltered life in the beginning.
One interesting plotline could be exploring Naruto's early days of training with Jiraiya in more detail. Maybe showing his struggles and how he overcame them during that time.
His presence could create a butterfly effect. Small actions he takes might change the future. For example, if he teaches a young ninja a new technique, it could change how battles are fought in the future.
One key event could be Naruto's time at the Ninja Academy. He was often alone and had no one to practice with, yet he still persevered. Boruto would see how Naruto's determination started early. Another event might be when Naruto fought against Haku and Zabuza. It was a dangerous battle that showed Naruto's courage. And of course, Naruto's training with Jiraiya would be important. Jiraiya taught Naruto so much, and Boruto would understand how this mentorship shaped Naruto into the person he became.
Probably his time at the Ninja Academy. Naruto was always getting into trouble there, but it was also where he made some of his first friends. Also, his encounters with the Third Hokage could be shown. The Third Hokage was like a grandfather figure to Naruto, and their interactions were important in Naruto's development.
Physically, Naruto can use his Rasengan or other powerful jutsus to fend off enemies attacking Boruto. His combat skills are top - notch, and he won't hesitate to engage in battle to keep Boruto safe.