Well, as of now, the Highschool DxD novels have approximately 20 to 25 volumes. The exact count can vary due to different editions and possible ongoing additions by the author.
The OC, as the Son of Sparda, could fit in by being a new and powerful entity. He might be initially seen as an outsider, but his powers would quickly draw attention. He could become involved in the power struggles between the different factions in Highschool DxD. For example, the devils might try to recruit him, while the angels might be wary of his presence. His unique abilities would also make him a valuable ally or a fearsome opponent. And in the high school setting, he could be a bit of an enigma to the other students, both human and supernatural.
The female characters from Highschool DxD, like Rias, would be intrigued by the unique personalities of the Naruto characters. Rias might see potential in some of them to join her peerage. And the Naruto girls, like Sakura, would be a bit shocked at first by the more revealing outfits and flirty nature of some of the Highschool DxD characters. But over time, they would bond over things like protecting their friends and the school. The Naruto boys would respect the power of the Highschool DxD devils and might even try to incorporate some of their fighting strategies into their own ninja combat styles.
The Highschool DxD manga has indeed finished. It wrapped up after completing all the planned story arcs and providing a satisfying conclusion for the readers.
In a Highschool DxD fanfiction, Naruto might appear if the author wants to create an interesting mash - up. It could be that Naruto is on a new mission and somehow ends up in the Highschool DxD world. For example, he could be chasing after a rogue ninja who has found a way to hide in that different realm. Or perhaps, the fanfiction writer just wants to see how Naruto's unique abilities, like his shadow clones or Rasengan, would interact with the magic and powers in the Highschool DxD setting.
These light novels are fairly popular. A lot of fans are attracted to the unique combination of high school drama and supernatural elements. They've also gained popularity through various adaptations, which has introduced the story to a wider audience.