Boruto fans have been waiting for a long time to see the protagonist using the Jougan but wonder why Kashin Koji has yet to inform him thanks to the latter's ability to see a lot of potential futures. However, the most likely answer is that the character can't warn him about the Jougan because he simply doesn't know about it, which could make a lot of sense given what this Dojutsu means for the narrative of the series.
The Jougan, on paper, could provide Boruto with a lot more control of Momoshiki's powers and also break the potential futures that Kashin Koji has witnessed, somewhat mirroring what his father did with the cycle of hate in the original series. Furthermore, it could also work as Ikemoto's way of putting the audience in a false sense of security when it comes to the destiny of the characters.
Disclaimer: This article is a speculative theory and reflects the writer's opinion.
Explaining why it makes sense that Kashin Koji doesn't know about the Jougan in Boruto

Likely, Kashin Koji is not aware of the existence of the Jougan because it doesn't appear in his visions, which makes a lot of sense. From a writing perspective, this Dojutsu gives the protagonist a lot more control of his destiny and the powers he has over Momoshiki, thus serving as something that could, on paper, allow him to save the day against their enemies.
Momoshiki taking over Boruto’s body has been a major threat in the story, which is why the Jougan (Boruto’s special eye) could play a big role in stopping it. This might reflect how Naruto, his father, ended the cycle of hate in the original series by overcoming something that seemed impossible.
The Jougan is also a typical battle shonen power-up, so for it to be satisfying, it needs proper build-up and surprise. Since Kashin Koji doesn’t know about it, its appearance in the manga could shock both him and the readers.
The contribution of the Jougan to the story

Incorporating the Jougan in the Boruto manga is something that the fandom has been expecting for a long time, especially because they already saw glimpses of it in the Studio Pierrot anime adaptation. Within the context of the Two Blue Vortex part, it is set up to be a major plot point when it happens since it is going to allow the protagonist to use his Otsutsuki powers without fearing Momoshiki's control.
There is also the running theory that it was Shibai Otsutsuki who planted the Jougan in him, which could be an interesting twist. The presence of Shibai in the story has been an endless source of theories, to the point that some believe that he is the real final boss who has been manipulating everyone, although that is still in the realm of speculation.
Final thoughts
It is very likely that Kashin Koji is simply not aware of the existence of the Jougan and that is why he hasn't informed Boruto of this detail. Furthermore, it has a lot of thematic value if he doesn't know and that element proves to be the decisive factor in this final battle.
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