Pot is laying him 3-1 and odds of him hitting on next card are 9 out of 47 times, or a little over 4-1, therefore making his call mathmatically incorrect.
This is true ONLY if you won't pay him off a penny if the flush hits. Most folks aren't that disciplined.
If he knows you are that disciplind and always lay down to a 3rd flush card, it also means that he can call you in other circumstances (say with overcards or a str draw etc) knowing that he can take down the pot both if either his genuine out comes or a 'scare' flush card hit (giving him 7 extra outs vs you usually )