Just to be a bit pedantic, he's estimated to have ~1.1M coins, which at current market prices would be worth $440M (9 figure wealth isn't a billion, but 'only' in the hundreds of milloins). Obviously he'd destroy the book on any exchange if he tried to cash out so it's probably worth much less than that.
The owner of those 1.1 million bitcoins can cash them out easily enough - without ever selling a single coin onto the open market. Walk into the offices of Andreessen Horowitz and tell them you want out. Those million bitcoins would be extremely valuable to anyone looking to build a large foundation of businesses around bitcoin.
I wonder how much you could cash out a month without really affecting the price. With that amount you could cash out half a million dollars a month for a very long time, assuming the exchange rate doesn't change much, and live like a king and/or fund the next project
