emilio's therapist says that emilio's personality is fixated at the earliest stage of development. in layman's terms, the therapist means that emilio's personality is at the earlier stage.

I believe the answer is: Stuck at the earlier stage.
This indicates that Emilio's would display some behaviors that similar as little children even if he's already grown up.
this type of disorders could be caused by both genetic disease (such as down syndrome) or psychological pressure that he received in his environment.