That's always an unconvincing argument. If you can't show him what he would have found had he 'looked into it properly' then no-one, apart from people who already agree with you, is going to believe anything other than you don't have anything to show. You need to think what you're trying to do in the discussion. If you're trying to convince people who don't already agree with you, then you won't do it the way you're arguing, because all you're doing is saying that they're wrong and when they don't meekly accept that, saying it louder. Of course, you might just be trying to signal to other people that already agree with you that you can be tough on the other side, then fine, you're probably achieving that, but it doesn't make any difference to anyone's opinion.