@Guspaz spoilers from novel related to why he doesn't demonstrate magic to his family (though the manga is beyond this specific part, the reasoning was just skipped)
"With all that being said, the fact that I’ll leave once I turn 13 is almost immutable. Understandably, I wasn’t fond of the notion of living with fools who use my lack of magic ability as a reason to discriminate against me, so I’m quite happy with how this has turned out."