Isn't there a phenomenon where molested children can suffer a degree of arrest in their lingual development? Like, word choice, complexity of speech patterns, even tones can just kind of lock at the age it happened? Not to say they don't understand, just that they stick with talking in a way in line with what's normal for the age it happened.Sensei should have been around 15 when Sekai got pregnant, but he also still acts, talks, and thinks like a child at times when he's around 31.
This seems a lot more common and severe with girls than boys, but it could certainly be believable that Akira was somewhat linguistically delayed, and especially why he monologues so much and was even capable of poetry when, but speaks so little and often poorly on the spot, considering how incredibly young he was when Sekai preyed on him.