@feha I'd say that his psyche not catching up with his new circumstances is quite believable. As cliche as the author and artist have set this scenario up to be, it's unfortunately quite a plausible scenario that can happen even in the real world ( a bit less dramatic and magical of course ).
Being the repetitive target of bullying can seriously damage to the ability to objectively view oneself as a desirable, capable, or effective individual. So even though you're correct that the lad objectively knows his abilities, one's trauma being much more outspoken than one's true value is something that happens far too often. Him being in an asian society, probably isn't doing him any favours either since some cultures just don't deal with the concept of mental health all that well. Which is a pity, since the psyche is so much easier to damage than to heal.
@lightning90 That'd require people willing to tell him. The modeling-girl, new school fellows, and the principal seems to be just the set up for helping him to get a grasp on his new OP status and a more believable way for the MC to escape his self esteem issues than for him to just snap out of it on his own. So far I'd say the author's done a reasonable job of managing this part of his character growth though it may not be as enjoyable or even likable