Kun?

Michihiro calls aki aki-san, so maybe michihiro is younger

"Kun is used by people of senior status addressing or referring to those of junior status, by anyone addressing or referring to male children or male teenagers, or among male friends. It can be used by males or females when addressing a male whom they are emotionally attached to or have known for a long time.Although kun is generally used for boys, it is not a hard rule. For example, kun can be used to name a close personal friend or family member of any gender."

Though it seems a tad too casual indeed lol. Maybe it's to pick on him a bit, making him seem like a lower status from him or something. After all, he asks him to call him simply Michihiro, and Akihito is just stubborn calling him Michihiro-kun lol.

-san is the most commonplace honorific and is a title of respect typically used between equals of any age. Less used between teenagers though, but it's very commonplace between adults. It just shows that Michihiro is a respectful person (〜￣△￣)〜

But he does say "Akihito" and not "Takaba". I'm sure we'll get more about this later.

The English translation is wrong. It's difficult to tranlate into English. The original Japanese is Akihito calling him Michihiro kun. But the monk said his name is pronounced Doukan. It's the same Kanji (Chinese character) but has two different pronounciations. Akihito is calling him that on purpose.

Thank you! Does this mean Doukan is younger than Aki?

Nah, I don't think Doukan is younger than Akihito. I think Doukan add the "san" after Akihito maybe out of resecpt for Asami (my guess). And Akihito calls him Michihiro kun maybe just to piss him a little, Akihito is not happy with his present situation, not knowing anything.

Oh, "kun" is part of Aki's bratty rebellion too. :) Thank you!