You missed the rather noticeable fact of complexion and went with the ears? LOL
If Grandpa were the father, a remarkably evidence-free assertion, then BO would be 100% white.
Per the ears, BO has 25% of his genetic material from Gramps, assuming the family tree is as specified. Plenty to account for ear shape.
We do not KNOW who the mother was anymore than we KNOW who the father is. Obviously, if Gramps is actually Dad, mom must be a woman of color. We don’t KNOW anything about this guy except what he tells us and now we know that you can’t believe much of that.