show him a gross over exgeration(sp) of iron sights, say like some five inch tall ones, then explain line of sight and bullet path travel/traj.

i had a budy of mine swear that "bullets rise" because of air when they leave the muzzle, i used the big sights and line of sight to finally get him to understand that the two points meet/intersect twice....

hope this help, cause i understand what your dealing with