Thanks for the (half) chapter, Chieri Project. Really, you should just wait until you have a whole chapter and release them as such. Piecemeal chapters appeal to almost no one (there's always the outlier).
I get it but the problem lies with the fact that he wasn't even able to make a reasonable guess despite knowing well that the girls would've grown and would be in Highschool by now. It just feels like the Author is forcing him to not function as a normal person. If he remembers them and thinks about them, then his first guess should've been one of the three girls, why the forced stupidity?
Others have said that MC-kun's mother and Miya are also responsible for the conundrum, and I think they're right--but only to a certain degree. It's not as if he's going back to a blank slate: it's his hometown, in which he can remember what's where and what should be but is no longer--in other words, what's changed. That said, he's been given a fair bit to work with:
・The girl's face already evokes vague senses of familiarity in his mind
・The girl isn't treating him like a total stranger and thus keeping him at arm's length, but instead is
very friendly with him--enough to play with him and wax physical, while continuing to subtly
seek his company
・The girl, once the topic of her name was broached, looked at him expectantly--and
he noted this himself, mentally
・The game the girl is playing indicates that he should be able to figure her name out
・His mother knows the girl, and will not allow MC-kun to cheat
In addition, all of this is happening in the context of his hometown--which allows for a lot of easy connections to be made. Speaking of Miya, her looks shouldn't mean anything: he himself already imagined how she must look quite different after ten years and the ways he expected her to change. Even if he was totally wrong, it'd make no sense for him to imagine someone so different from the actual person as to be someone else--unless he had cause to imagine such, like knowledge that she had in fact changed unrecognizably (e.g., she'd become obese).
These things should click in his head or the author won't allow them to for as long as it's expedient for the story he wants to tell, which is par for the course with not just romcom stories but just about any that isn't completely serious: it's a useful tool for padding, but it blatantly marks a lack of writing skill and/or laziness.