Simply ask him what is keeping all the permit holders carrying guns now from gunning him down? The exact same people will be carrying exactly the same guns!!! The only thing that will change is that some will wear their shirt behind their guns instead of over their guns. That is the ONLY difference!
Also, since he doesn't like guns, he should insist that no guns be allowed to be hidden, that way he can know exactly where they are all and run away from them. Is it not, to him, more devious that a person feels the need to hide their guns like they....well...have a reason to hide it?
Anyone who says, "I support the 2nd amendment, BUT"... doesn't. Element of Surprise: a mythical element that many believe has the same affect upon criminals that Kryptonite has upon Superman.