Best bet if you don’t want to face reality, is just to say “I don’t think it’s an issue” and leave it at that
To give legitimacy to his lack to present proof, in fact, his resistance to provide proof, only encourages such behavior and makes your potential supporters the objects of ridicule
IOW, just SHUT UP if you don’t want to go there.
Or, considering that he is a lawyer, "I haven't seen any evidence to convince me one way or the other."