Healing a Puppy... His mother stepped on him.

His mother had her pups in our carport 7 weeks ago. Last week all was well until we heard a puppy screaming outside and found this little one crying in pain. Our best guess is that mama stepped on him. The day before the accident he was a happy, playful pup with his 6 siblings. Now he can't walk on or move his back legs or his right front leg. He's eating, pooping and peeing as normal and loves to be loved on with petting and belly rubs. He lives inside with us now as his mom wants to separate him from the liter.  He just lays in one spot. My husband and I are retired and all of our kids have moved away. We are able to sustain a comfortable, simple life, but are in no way prepared to foot medical expense of this magnitude. The initial vet visit alone will be $100 . After that we are looking at x-rays a possible surgery. We just can't afford that.  If I can't get him the help he needs within the next week, we will have to have him euthanized. He is in pain. This is not the quality of life any pup deserves. Thank you so much for any help.