That assumes that Barry, jr., knows who his father is.
Maybe the Dunhams didn't want to let Frank M Davis into the family (for which there are a number of possible reasons), so they just "assigned" him to the far away foreigner because while still a bit messy, it was much easier to deal with. Think of the internal family dynamics when Grandpa's drinking pal (which is what Dreams explicitly says he was), gets the daughter pregnant. Big family crisis. Lots of embarrassment, followed by "Hey Frank, how about you move back to Chicago?"
Pop
Sitting in his seat, a seat broad and broken
In, sprinkled with ashes,
Pop switches channels, takes another
Shot of Seagrams, neat, and asks
What to do with me, a green young man
Who fails to consider the
Flim and flam of the world, since
Things have been easy for me;
I stare hard at his face, a stare
That deflects off his brow;
I’m sure he’s unaware of his
Dark, watery eyes, that
Glance in different directions,
And his slow, unwelcome twitches,
Fail to pass.
I listen, nod,
Listen, open, till I cling to his pale,
Beige T-shirt, yelling,
Yelling in his ears, that hang
With heavy lobes, but he’s still telling
His joke, so I ask why
He’s so unhappy, to which he replies . . .
But I don’t care anymore, cause
He took too damn long, and from
Under my seat, I pull out the
Mirror I’ve been saving; I’m laughing,
Laughing loud, the blood rushing from his face
To mine, as he grows small,
A spot in my brain, something
That may be squeezed out, like a
Watermelon seed between
Two fingers.
Pop takes another shot, neat,
Points out the same amber
Stain on his shorts that I’ve got on mine and
Makes me smell his smell, coming
From me; he switches channels, recites an old poem
He wrote before his mother died,
Stands, shouts, and asks
For a hug, as I shink, my
Arms barely reaching around
His thick, oily neck, and his broad back; ‘cause
I see my face, framed within
Pop’s black-framed glasses
And know he’s laughing too.
— Barack Obama (Cashill says it’s written by and about Obama’s real father, Frank Marshall Davis)