I don't know much about this, but it appears that no-one else has any ideas at all, so I'll have a try.
--Culling the bird is always an option, and is sometimes the best or only option.
--I have seen threads with chickens who had injuries that looked this bad or worse, and they survived.
--Separate this bird from the other chickens, so they cannot peck it any more (you probably already did this.)
--I cannot tell if the vent got pecked or not. You can look at the vent on a healthy chicken, and see how far it is below the tail, to get an idea of where to look on this chicken. Or if the chicken poops while you are watching, you will know where the vent is.
--Provide fresh water and the usual chick starter for the chick to eat. You can also add water to the chick starter, to make a mush that chickens usually like. For wet feed, replace it at least once a day so it does not have time to go bad. (Give the leftovers to your other chickens each time, rather than wasting it.)
--Given how long has passed, I'm guessing the bleeding already stopped, and the area probably has a big ugly scab.
--I've seen blu kote and several other things recommended in the various injuries threads, to help keep the area protected and moist. But I've also seen recommendations to leave injuries alone and uncovered. So I don't know what to suggest here. (I've also seen suggestions for bandaging, and for not bandaging.)
@azygous @Eggcessive Any treatment suggestions here? Spread something on it or not? If so, which something? Bandages or not?