It really depends on how much of his coins he spends on how obvious it is that it is him spending them. If he spends only ~50 of his coins and doesn't touch his others then the market would likely not even react, especially if he spends the coins he mined later rather then earlier. If on the other hand he spends 500k of his coins then yes the market would likely crash as it would not be able to handle this kind of selling pressure.