See any grass stains, mud, blood, anything? Looks like he just washed it.
Why would you? Was the ground moist? hard to get grass stains or mud without moisture? Was the struggle even in the grass or on the sidewalk?
Why would there be blood on the back of the jacket? The injuries were to the head. If the injuries were caused by his head hitting the ground, the back of the jacket would have been against the ground. Why would you expect blood on it?
It's clear from the pictures that while the head blead a bit, that it was dry before hitting the jacket. Not to mention there is a collar to the jacket and if any blood reached the jacket, you wouldn't see it because it would likely deposit behind the collar.
Are you expecting huge stains of blood? There could be small starts of blood mud, or grass stains on the jacket. Without closer examination you can't say there arent.