Shouldn't he have friends that would know a lot?
However, there are a couple of points that support his theory. First (and most obviously), the amount of time bin Laden has spent away from the spotlight. By my count, the last time he was videotaped was last October, not long after the 9-11 attacks. A meglomaniac like bin Laden likes the spotlight, particularly when he's being challenged. It is very uncharacteristic of him to go eight months without offering a "fresh" videotape, counteracting U.S. assertions. The two most "recent" videos, which appeared late last year and early in 2002, were clearly recycled from an earlier taping session. Watch the upcoming 9-11 anniversary very carefully. If bin Laden doesn't issue a new tape crowing about his "achievement" and continued survival, it will be another sign he's dead, or effectively out of the game.
Another clue about bin Laden's demise came in the letter taken from Abu Zubaydah. For the first time, it spoke of an Al Qaeda without UBL, another indication that bin Laden expected to meet his fate in the near future. BTW, has anyone hear a follow-up on that report out of Afghanistan two weeks ago? A set of remains had been found, and there were hints that they might be bin Laden. Interesting to see what the DNA testing will reveal.