Keep in mind that at at best, this is merely anecdotal evidence. (The same sort of evidence that us skeptics complain about when someone says "I don't need to get vaccinated because I never get sick", or "A psychic predicted my future".)
Its possible that full body scanners are completely useless (as his story suggests). Or its also possible that they are the most effective security technique in the history of the universe, work in 99.9999% of cases, and he just happened to be the example of the 1 in a million case where something slips through. At this point, we just don't know.
And something else to consider: Remember, the 9/11 hijackers managed to make it through security supposedly with box cutters even though airports had metal detectors and other techniques. Does that mean we should just forget about all security measures? Would you feel just as safe flying if there were absolutely no security before getting on a plane? After all, if anecdotes were "proof" that a particular security measure is ineffective, then metal detectors and other security personnel would be eliminated years ago.
To really say whether the body scanners are effective or ineffective, we'd have to run some sort of large-scale survey/test, where we send a large number of people with planted weapons through security at random, and compare the number of people who are caught by the scanners with the number caught through other means (metal detectors, observant security personnel, etc.) Until then, we're just relying on anecdotes.