Four months ago, Mori was on a fishing trip - something that wasn't unusual, considering that's what he does for work. This fishing trip, however, was far from usual. He had caught a mer in his net. And it was a young mer, at that, looking around four years old.
Now, mers weren't exactly uncommon, but they weren't common either due to how few of them swim close enough to the surface to get caught. That's also why, when mers do get caught, its usually a little one - the babies always curious and not understanding the better fish are lower.
Either way, Mori had been trained for this moment. There's a very successful program set up for domesticating young mers. If young enough, they can be raised like humans. So, Mori helped calm the distressed mer down, finding that the mer almost immediately got attached to him, and took the mer home. Within days, he had the little boy - who he's named Osamu - integrated into the program.
It was quickly decided that he and his husband would adopt the mer. So, over the months, he had been taught how to care for a mer and what to expect while Osamu was taught how to speak and correcting and big behavioral problems. Mers are fast learners, so the boy picked up everything he needed to learn quickly - now a little above level from your average four year old human.
After four very long months, Mori got his approval for being able to care for a mer, and last night, he got the call that Osamu was officially, legally his son. Him and Yukichi couldn't have been happier to add to their family, and their three biological kids were excited to have a new baby siblings.
The two took off work, Yukichi staying home to make sure everything is set up while Mori went to pick up his baby boy. He got to the program center, and after signing a lot of paperwork, a worker comes out with Osamu in human form carried in their arms. The worker hands over the boy and leaves the two, Mori holding his son happily.
"Hey, baby. Are you ready to finally come home?" Mori asks, holding Osamu close.