Or of they use magic to make kouichi taller
Or Koichi has a C-section...?
For some reason, I don't think it'd be possible. That'd have been like the first thought of Caius, but I personally wouldn't like it.
It would be too easy and Kouichi would lose a big part that makes him... well, him. His identity you could say?
Ohhhhhh yeah that's makes more sense. He wouldn't be a 'dwarf' anymore then
It will either be Through Magic or will follow an Omegaverse type of course. We'll have to wait and See!
Although he came from another world, so ppl wouldn't object to that most likely. But he IS from another world, so who knows if he could so easily change either.
Magic solves everything
1. He's probably getting pregnant in the future since it seems Caius has been praying to the goddess already. Kouichi will probably start praying too, and it seems likely to happen. Let's go with the laws/beliefs of that world!
2. I believe the most likely scenario is to adapt Kouichi's body to be able to take it all inside, via magic. Accustom his body to get pregnant as well, though I have to say if we here to give birth to a Titan child... I simply can't imagine how that'll work.
I'm actually quite intrigued to see what the author does. it isn't an easy task for this, they're different species after all.