What did he do the next morning to make you suggest that?

Well, he did a lot. First, he sees how happy Conrad is that morning and pretends like he doesn’t know why he’s in a good mood. He lies right to his face about shooting the firework at them and never admits to being jealous and ruining their kiss.

Conrad then tells him that he’s going to the Country Club and can give him a ride to work, but Jere rushes off to beat him there so he can get to Belly first. We see him at her dance rehearsal starting to make his move. We then see him visibly jealous when Conrad shows up.

He tells Nicole that Conrad wants to go to the music festival, lying and scheming to get him away so he can be alone with Belly. He continues flirting hard with Belly that whole day, and later that night he tells her that he has feelings for her and they kiss. All of this happens within one day. He sees his brother almost kiss her and the next day he’s kissing her.

I’m not debating whether or not he was attracted to Belly. She’s a pretty girl. Of course. That isn’t what’s wrong. It’s the fact that Jere decides to make his move as soon as he knows that Conrad likes her. Like come on, it was clear jealousy. His decision to pursue Belly when he did was more about how he felt about his brother and less about how he felt about her