Maybe he’s just a homosexual and didn’t like girls
There's no big reason for him falling in love, I think Kaede just grew on him. Usually he never really looked at the person he was dating, but Kaede already got him curious at the beginning since he never contacted him. Then he got intrigued when Kaede said that all the things that made people call him a scumbag was cool. And now that he really looked at Kaede, he started to think he was cute. It was showed by him thinking that Kaede was like a puppy. He also never met anyone who was as considerate and understanding as Kaede before, which caught him off guard and made him want to be nice to him and understand his way of thinking. At this point he already couldn't keep his mind off him, and along the way his feelings for him grew.
I kinda wish there was more of a reason for why he liked him. Cuz like rn it's just because he didn't pay attention to him and respected his boundaries. In a way its kinda the bare minimum. Like he just literally acted the way you should in this type of relationship and then he just fell for him.