Here's my theory. The man worked in the office whose window faces the ledge. He stood on the window sill and pulled himself up onto the roof for a better view of the burning towers. Note that (1) the window sill seems high enough in relation to the roof that a reaonably limber person could use it for roof access, and (2) the buildings to the north probably would not have blocked the man's view of the burning upper stories of the WTC.
The South Tower collapsed a few second before this picture was taken, and huge smoke clouds began billowing toward the roof where the man stood. He jumped back onto the ledge, which probably is wider than it seems in the picture, frantically trying to get back into the building. While it's hard to tell for sure, it looks as if his legs are bent, as if he's just jumped down from a height of a few feet; most likely, he hung at arm's length from the edge of the roof and dropped to the ledge.
One thing that would work havoc with my theory is a closed window. Certainly, the man would not have closed the window behind him when he climbed onto the roof.* Try as I might, however, I just can't tell if the window's shut, so I'll say that it is open and the theory is still possible. Actually, even if the window really is closed, it might have been shut by another person in the office who wasn't aware that the man was on the roof.
I decided to test my theory as best as I could during lunch hour. I went to the area and got a pretty good look at the ledge in question, from ground level of course. The green-roofed building is 47 West Street, a sort of low-budget office building, what's known as Class B or C. As I had suspected, the ledge is much deeper front to back than is apparent in the picture, at least six feet, and getting from the window sill to the roof looks quite do-able. There's actually a waist-high iron railing around the ledge that doesn't show up in the picture, it's old-looking so it probably wasn't added post-9/11.
The one possible weakness in my theory is that it's difficult to tell whether the man could have seen the burning towers from the roof of 47 West. The brown building to its north, an apartment tower with a Washington Street address, is much closer than is apparent in the picture, basically looming over the smaller building. Even so, it's at least possible that the man had a view - or maybe he thought he would, when he went to the roof - so his presence there would not be surprising.
"Let me think about it" = the coward's way of saying "no"