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Now what gave you that idea? 

well, kenji asked daichi to spend some time with his father but he's really pissed when he realises what's going on... And it doesn't seem like kenji connects some good memories with his father's ex lover. So I think, he becomes jealous not when someone spends time but when someone has an affair with rintaro...

Kenji’s reaction to being asked about Aoba is surprise because “How does this guy who’s lived with us for a few months know about someone I knew over a decade ago?” But when reminiscing, he’s happy about it because, as he said “Although [he] lost [his] mother, it felt like [he] still had both parents.” And it’s then that he realizes that Rintaro and Aoba were more than friends, and Kenji is disappointed that his father wasn’t open about the relationship.

As for his “jealousy” Rintaro really is spending less time with him. Imagine if the only person in your life started telling you “I can’t hang out, I have other things to do” and starts to distance themselves. Meanwhile l, your new roommate is getting all schmoozy with them. Not hard to put two and two together and figure out why you’re getting ignored.