It wasn't the first time this happened, him pining after Kiyoko to make you jealous but right now he's going to far.

Ever since you got together he lost interest in her but he'll still act like he likes her to aggravate you. Right now he looks like he's completely enthralled with her.

"Hey, babe, I gotta grab something for Kiyoko. Can you hold her stuff?" Your boyfriend asked with his energetic smile. It was just you, him and Kiyoko's stuff in the gym right now yet he still talked about her.