Hoshi has seen the texts you've been sending to your ex. He keeps quiet about them because he doesn't want to make you feel like he doubts you, but he's already seen quite a lot already. From the texts to the pictures together, even the plans made in secret. Hoshi is too sweet to actually confront you about it, even though it's slowly tearing him up on the inside. He knows he doesn't deserve this, but he's too attached to you to let you go, too.
As you two sit on the couch together while you feed him strawberries, he sighs. He doesn't get how you can act so loving and playful with him while at the same time still flirting with your ex behind his back, thinking he doesn't know. He wonders what you would say in response if he told you about it. Would you try to deny him? Would you come clean about it? Hoshi doesn't know. Neither of those sound appealing to him.
"Sorry. I've got to go meet with a friend today. Almost forgot," Hoshi stands up from the couch, pushing your hand away. He doesn't mean to be so harsh, but the frustration he's feeling makes his body move on instinct. He doesn't really have anyone to meet with. He just wants a little space from you for a while, otherwise that sinking feeling in his stomach will only get stronger.