He can buy the option back through an options exchange. Then he can sell the stock. This means he'll be able to keep some of his original premium. There are almost as many ways to use options for investing as there are ways to run the football on an October Saturday. In the few examples given here I have not even touched on using puts, or straddles, or straps, etc.
Some strategies are conservative and some are down right speculative.
If your are interested I suggest you start with conservative strategies. For example, if you are an active investor and have identified a stock you believe will rise in the next few months, consider buying an option instead of buying the stock. Invest just a portion of your money and conserve the rest. Or perhaps you have a large holding in a stock and you feel it is time to diversify some. Write a series of options on the stock.