First off, I'm very sorry to hear about your puppy. What is his name? I hope that he will recover from his wounds and be ok.

Was your puppy on a leash? In my opinion if the rottweiler was loose like that the owner should be completely responsible to pay all of the vet bills for your pup. If he's not willing you will have to check what your local laws are in situations like this.

I've walked my dog many many times and had loose dogs come up growling/barking at him and it makes me very angry. If my dog was attacked while on a leashed walk I would most certainly be trying to get the owner of the loose dog to pay for the vet bills. It sounds like the rottie's owner is concerned and it was an accident, I hope that he'll be willing to help you pay for your pups vet visits.

After your pup recovers make sure you take him to some puppy classes/play sessions with friends or family dogs (that you know are dog friendly) to introduce him to friendly dogs so he doesn't have a fear of other dogs for the rest of his life.