I think it's because that asshole Masaki punched him and then deleted all his numbers out and told him not to have any number in there but his and his Dad's.
I figured it was because he thought he didn't think he was gay and didn't want to waste his time, but yikes, maybe.
Marauder might be right That sounds like the most plausible reason to me rn.
they already broke up, his dad's dead, and like he said he doesnt give af, so it wouldnt make sense why he wouldnt give his number just cause masaki told him to. so im just gonna assume he didnt think theyre relationship would develop thus he didnt wanna bother giving out his contact info
His Dad didn't die until his second year in college, which is when he broke up with Masaki and dropped out of school. He tried to get his contact info two years earlier at their graduation.
https://www.mangago.zone/read-manga/ore_no_oniku_chan/mdx/d_chapter-5/5/
He never explained why he said "I don't wanna!"