And this is important because...
All he had do was pick up the phone and call his grandmother. She had to know.
How do you know she told him the truth? Ever consider the possibility that she blamed his father for her daughter's unhappiness, and wanted to portray him in the worst possible light to her grandson?
What unhappiness? She finished college, got an MA and PhD, and did what she wanted to do. She traveled all over the world. I don't think BHO Sr being gone bothered her all that much, she might have regreted the loss of Lolo more, but maybe not. She didn't seem to really like men all that much anyway. (I mean as to have around underfoot as it were.) She does seem to have tried to shelter Barry from the stigma of being fatherless, and "different".