Even though his arms are folded, I would guess the backpack is on the ground at his feet.
If you look at the race clock suspended over the street, this photo is even earlier, at 2:18 of the race.
I think the photo of him with the backpack on his shoulders and the crowd has thinned out is at a later point in time and when the two men have moved up more toward the finish line to the area where the bomb was left. If that is correct, the backpack must be around there somewhere in your photo.