It just occurred to me: Father and Shawn are not the same person. Shawn was taken away and rendered down into little tubes of Baby DNA that was subsequently used to put flesh on the Type 3 synths. Father is some ageing psychopath who saw a dangerous but mentally vulnerable mercenary somehow break into the Institute and decided it would be easiest to manipulate them into joining the team. He was about the right age, so he poses as your lost son. He's about the right age to give it creedence, and because the player character is too stupid to find Abernathy Farm on the fifteenth attempt without Preston Garvey marking it on her map, he believes it.
Father gets a replacement for Kellogg (who he honestly hadn't expected to lose) and a weapon against the other factions. As his ill health progresses, it occurs to him that his best chance of preserving his policies beyond his death is to persuade his supposed parent to mindlessly support his edicts. He doesn't have to understand, just make it so that the department heads are terrified of the consequences if they stray from the path ordained.
And because the player character is an utter moron, they buy into the whole thing uncritically.
It explains so much!