What event is this?? It couldn't be the Boston Marathon itself. And I don't think anyone so severely injured in the Marathon bombing could be in racing condition so soon.
What I suspect is not a hoax but someone genuinely impaired whose cast or bandages or brace was being shaken loose and at risk of tangling with the wheels or falling off. As a matter of fact, it could have been the blanket over his legs that was falling off and at risk of tangling with the wheels.
That is not a racing wheelchair, and in wheelchair marathons and races the wheelchairs are run by the occupant's own hands, not with someone pushing the chair. I don't see anything in this film that suggests that he had lost both legs or even one leg. I would think that if he had a prosthetic leg, he'd at least try walking on it.