I don't think he's homophobic at all. It isn't homophobic to want his friends to avoid what he knows to be heartbreaking rejection and judgments from others. He thought Mikami was just confused and that Sato was straight so he acted sternly to spare them from a messy situation. In fact, it would've been much easier and more selfish of him if he had just happily encouraged them to be together to hell with consequences cuz then he wouldn't be the only gay in their friend group and probably school and he'd be more understood too. But because he's a real good friend he didn't do that and instead took his friends interests at heart and tried to do the right thing. It actually made me really like his character.