Don't know your setup at all, but if it is a bite, what is in his habitat that would be biting him? If it's a paper cut, you use newspaper as a substrate? If so, I doubt he could get a cut by a piece of paper sitting on the ground unless you have a water dish right beside the edge of a piece of paper which would keep it from moving. Looks more line a scrape to me, do you have anything with a rough edge that he could have caught it one while moving about?