Well he still had the other brothers and, even if they were younger, generally a person w/ a disability was seen as lesser than others. Having an heir w/ a disability could be seen as weakness on their part, so the father decided to throw him away. Tbh I thought they would've disowned him, but he didn't. Disowning a child is seen as a bad child who was raised by bad parents, so they couldn't disown him. "Throwing him away" was their best option w/o showing weakness.
Mmmm. But it was his OLDEST son... That is a BIG deal in Confucian countries. Disabled or not, if fate gives you that particular son as the first, it's that the gods have decided over it that way.
In Taisho era, people still thought very much that way. This mangaka hasn't done her homework right.
Still, since there aren't any new episodes, we'll never know how it ends...
It may provide a good starting theme, but I'm pretty sure no Japanese father would "throw away" his oldest son like that, not even in the Taisho era.
When you read manga, you meet a lot of un-japanese bad parents!
Still, nice story.