Ask him flat out whether he would feel safe flying if the checkpoints reverted to their pre-TSA state of metal detectors and bag x-rays. If he pulls the 9/11™ card, describe the changes that will prevent another 9/11-style attack from recurring. Explain the financial costs of AIT and that it is actually less effective as a security tool that old-fashioned metal detectors. Explain that AIT slows down the lines, and that by amassing more people before the actual checkpoint, AIT is putting him at a greater risk of someone choosing to attack the actual checkpoint.
If he still fails to understand, then give up.