Edit: oh wait- I didn't mean- just- sjjsjjdjjdjjsjdjsjdjdjdj
(I had made this comment before reading the other comments on this page lmao. I'm just flustered here because Shigeo from MP100 is heterosexual and I didn't mean to sound ignorant lol)
That said, I suspect Yuuri is autistic spectrum first, and aro/ace second. It's frequent for people on the former spectrum to have a hard time grasping or expressing abstract concepts like emotions, and this seems more like Yuuri's situation here, especially from the last page.
You guys are so quick to just slap some kind of label on the kid. OmG hE mUsT bE aCe Or AuTiStiC!!
NO.
He is just a highly observant younger brother while not being very observant around himself. I've seen his kind character more then a few times while reading manga. They are there more or less to add clarity as well as comedy. End of story.