Kana is best girl for Aqua (Miyako is still Best Girl overall).
If she was a little bit smarter in terms of relationships, Kana would realize that Aqua basically confessed to her on their mini-date playing catch. Kana is the only girl he feels actually comfortable with. He's not friendzoning her, he outright told her she's the one he actually trusts with his true feelings. Yuki or Ruby would probably recognize it instantly if they witnessed it. She playfully teased back about the preference for older girls than him comment, but I'm pretty sure he was serious, and I don't think he was referring to Ai. He looks to the girls in his class as potential friends for Ruby, with his dispassionate eye. But he pays attention to Kana. Kana is a pessimist and misinterprets their entire little date.
Sure he gets flustered around Akane, but he also has no idea what kind of relationship he ultimately had with Ai. He was a fan, then he was her son. Was it ever a romantic longing? Did his time as her son kill the psudeo-romantic vibe fans have with idols? Did he develop an Oedipus Complex? He gets mixed up seeing Akane do the Ai-act that Akane thinks he likes, but that's because he's never fully settled on what his relationship with Ai was, he never got the closure either.
But he never really relaxes around Akane, even if that's what she desires back from him. He always does and always has relaxed around Kana.
Lastly, why would Aqua even go on a skip-school date with Kana before immediately getting into a professional fake marketing relationship? Could be that he wants his first date to be a real one, and his line at the end of the scene all but confirms it.
The whole mini-date was a test to confirm that he isn't attracted to Akane that way...well, you can't test against one thing without a genuine control sample.