This cat definitely needs to go to the vet. There is no natural equivalent, a vet needs to see the kitten. I'd do it as soon as you can, the longer you wait, the worse his chance of ever walking. If his hind legs are both broken, he will never heal unless you get him to a vet that can set his bones.
Afterwards maybe it would help to get him to another mommy cat (I don't really know if another mother would feed him?) but... no use to do that if you don't get his bone set.