He wouldn't need to buy all of them. If his purchasing streak pushes prices up so high, or so fast, that nobody will sell them or use them, then for all practical purposes, bitcoin is dead. A currency with no current is not.
On the contrary, as the price of bitcoins goes up people will be more inclined to use them. People will offer steep discounts on the items they sell to get the bitcoin in the hope those bitcoins will go up in value even more.
Right. The stores confirm this. Bitcoin highes are rush hours on bitmit and others.