Not likely that the kid in the picture is the 8 year old. An 8 y/o would be pretty big to be sitting on someone's shoulders and his dad was running in the race. He was with his mother and two siblings.
You are correct. Also, his father has a full head of hair, not partially bald.
Notice the female in the white jacket beside/behind the backpack guy. In the photo you posted, it appears she is with him- look at where her arm is and how her body is pointing toward him. You don’t stand that way next to strangers. She’s next to him in the other “before” photo, too. And if you look carefully, in the “after” photo, there is a white sleeve just to the viewer’s left - i assume that’s her, too.
So either the suspect is irresistible to strangers, or he brought along his girlfriend to bomb the Boston Marathon, or he’s just a dude watching the race. My guess is he’s just a dude watching the race with his girlfriend or a friend.