I actually hope the author leans hard into the self-indulgent sulking and misery of it all.
Going from "having to help Sayaka with everything" to being surpassed by her--first in martial arts, and then seemingly in every facet of adolescent life (academically and socially), would arguably do a number on the ego of a pubescent boy, based on those I knew growing up. Deciding to "take his ball and go home" over it isn't really that unheard of, nor even all that alien of a reaction--especially given how much Sayaka seemed to depend on him all that time prior.
Him acting super sullen toward her, snapping, being short, avoidant...would all be honestly in-line with what we're shown as the catalyst for the shift in his demeanor and day-to-day; I can't say it would be good, character-wise, but it'd also establish a clear baseline of where he's at, and how far he needs to go to grow as a person and come out the other end as a reasonably-adjusted young adult who actually deserves the kind of friend that would stick by his side, even as his own mother has seemingly had it with him.
Sadly, I do not think we'll get a proper pity party about it, though, because the first chapter had Kazuki freaking out over discovering that his childhood friend is actually a hot sexy babe, and every interaction between them since then has been him fighting the hormones trying to hijack his brain. If he were truly resentful and sullen about what had transpired, he'd be more angry, I figure, than anxious & frantic. That tells me that he's not resentful toward her, even as she represents (even in part) the reason he gave up.
Perhaps it'll be a matter of "Kazuki knows it's not Sayaka's fault in the end and thus can't bring himself to resent her". Which is actually remarkably mature of an outlook, given everything else we've seen about him, but I admit I want the melodrama of him being a petulant child, so that the transformation into a halfway-decent Kazuki at the end of the series feels more satisfying. Purely just me and my own tastes, of course, but it would be far heavier than it would appear the author is intending to go, so.
But at least the open contempt that his mother shows toward him makes sense, now. If he did simply quit without telling her the reason, then she'd be understandably upset with (and seemingly even disappointed in) her son. If she does know the reason, then I suspect she thinks it's bullshit (which, agreed), and that he's being a dumbass (again, seconded) and wants Kazuki to grow the fuck up a little or at least not continue throwing his life away. (She's maybe a bit harsh, but if it's been like this for a year, then she might have simply run out of patience, which....yeah, understandable.)
Can't say I'm super surprised at this development, but I'm just glad we have a concrete reason for why Kazuki is the way he is, now. It's even understandable as to why Sayaka still seemingly likes him and wants to be around him--he helped her all their childhood, and I wager she might not even know the reason he quit the dojo (which makes me think his mother doesn't, either). But he's only ever been helpful to her, protective over her, and a friend to her, so that's all she sees, and she is attached to him as a result. She might not have romantic feelings (or at least not be aware of having them), which makes this attraction one-sided from his end; though I hope that more than any sort of romantic progression, we see character progression for Kazuki.
If he comes out the other end of this having grown up a bit to the point his own mother isn't embarrassed being seen in public with him, I'll consider the author having succeeeded, whatever the situation between him and Sayaka.