Well, the story doesn't tell what the dad was thinking when he was buying the shares. Maybe his idea was to support his son and sell the shares back to Sivers when he has made enough money, for modest profit. And maybe he considered any profit scenario unlikely^1. But then he forgot about it and maybe Derek forgot about it as well. However might be weird that Derek didn't know that he was selling shares of his company. After all company shares/stocks are a concept that even 10-year olds understand at some level.

[1] In my experience people consider starting entrepreneurs likely to fail. So it might be as well that FFF round is considered a gift from the investor.