Looks like the back leg has a compound fracture. Unless you take it to the vet its most likely outcome is a lot of pain before it dies. I have seen animals survive from such injuries but the majority do not. If I found it in my yard I would immediately euthanize it.
Yup...that is a serious leg break and, unless you find a vet that would go to the trouble of treating it and feeding/caring for the youngster until it's fully recovered, it spells doom for the little one, with much pain and infection and suffering. Sadly, I suspect the answer is euthanasia and a solid stone to the head is the quickest way. Don't try and wring the neck, or you might find yourself holding a head in one hand and the rest of the bird in another...don't ask me how I know this.
If the parent birds are still interested in the little bird you might be able to get the parent birds to look after the little one. It's a slim chance but would be the best outcome for the bird. The biggest issue would be to protect the bird from cats and other predators. If you had a shed or something nearby you might be able to open a window wide enough for the parents to enter but keep the cats out the little one could stay in an open container in there and the parents bring it food. We have done this with Spotted Towhee but the chick was the last one of the clutch. The parents found the bird by its cheeps.