There’s this man who exists and he has done nothing to get to know his own son on the son’s terms, instead of just how he sees that son through his own eyes. It does not occur to him that this son is so different from the eldest that he simply can not approach him in the same manner. Eyes to see this son’s activities will only reveal a cursory glimpse of a human being trying to stay alive - somewhat unsuccessfully you can imagine his interpretation being - whereas observation of the eldest reveals immediate stimuli that can be engaged with.
The youngest son’s world is not open like that, but that is not to say it is closed. Far from it. The youngest son’s world exists behind layers you must traverse and become acquainted with not through observation or action, but through listening.
That is all it takes. Listening. Simple. Any father can do it.