The guy gets bashed on a lot by his schoolmates up till now that I'm not surprised he didnt notice an actual girl was legitimately trying to flirt with him. Not to mention Maho tended to behave oddly with everyone up till now so the sudden increase in interest was unexpected.
@HYBRID_BEING
Here is the panel in question in the raw
It would be "ni, san... ju nichikurai..." My Japanese isn't near as good as the translator's but nii and ni are mostly different. Still the pause may have the unintended effect of sounding like nii-san.