There is a lot of talk about age in this chapter, so I’m gonna take a moment to give a few words.
Kai is 18~19 at this point. That’s above the age of consent, though at the time of publication there may have been a difference for same-sex relationships.
”Adult” in Japan is 20. That’s a hard limit, socially and in many other legal aspects.
It’s considered unacceptable to be in a relationship with someone under 20 if you’re over it, currently. At this time it may or may not have been so, though I didn’t see that in popular-culture media.
While I do think the age difference here and Kai being under 20 is somewhat problematic, I wouldn’t do anything to prevent such a relationship, there or here (I’m located in the USA).
I would, given the opportunity, have a discussion about it with Eisuke. Once the situation were fully explained, I would say “well, that’s fine, I guess, but you’re right on the line, there, son.”