When he sold his house the price was around 600$. He is underwater, but not yet in the danger zone.
However when the price will drop below 300$ he might feel an irresistible urge to cut his losses and may sell close to the bottom.
If he won't panic sell, he'll still have to wait several months for the price to rise back to 600$.
It is a game of balls. How big are OP's balls? Can he hold breath underwater for a year? Thats the big question here.