I have done it while grooming angora bunnies and it does heal quickly. I have a dog whose butt gets like that. It is a hard job but the hair and poo must be clipped. I try to clip the butt of all hair to avoid the problem but am not always successful. When you make them bleed it makes you feel like a monster because they love us and depend on us for their care. Give him treats and love, he will forget. Keep the neosporin handy, or bagbalm, whatever.