Tell him that if he is still serious in 6 months, you'll give it a try, assuming you haven't met someone else. If he's the type who can't stay single, who can't wait without the validation of another woman, he won't stick around. If he realizes he was an idiot and you were the one the whole time (which, lets face it, is unlikely) he will still be pining for you in 6 months. If he can't take "the risk" that you might not be interested in 6 months then he's not worth it.