@screamdoom well, cutting off the horn is like branding a criminal, like Jack Sparrow's brand in Pirates of the Caribbean. So, it may not get better within a human lifetime, if at all.
On the other hand, maybe demons shed and regrow their horns on a "regular" basis like deer.
@TheFiller
I'm not sure how much of it was covered in the manga, but let's just say he's supposed to be a very, VERY strong adventurer, with a lot of social ties.
Code:
Mod Note: Please use spoiler tags when talking about chapter-specific or future events.
@Solipsist I'm still worried though. Last time someone persecuted Latina for being a demonkin, she tried to cut her other horn. It couldn't have been good for a child's mental state