"Abaddon and the vessel are both sentient and coexist within the vessel" "the vessel talks to Abaddon and wishes for things, they're friends!"
Ok but what if the vessel (yk the boy) has passed away ever since the cross was burned upon the vessel by the priest. BUT abaddon was originally much more mature and demon like (more like a actual adult) near the beginning of the possession. (Aka the first few decades of being inside the vessel) But as time went on and hundreds of years passed abaddon slowly got more and more used to the child form. Over such a long to Time of being treated like it abaddon subconsciously developed childish habits (without quite recognizing it due to not being knowledgeable in human customs) due to the way people around him treated him.
