My kitten can't move his back legs! PLEASE HELP!!!!!!!?

A couple of days ago I found a kitten in the ditch by our highway, so I decided to keep the little fella. For a couple of nights, he slept in the garage and got really fat, drinking off milk and eating crushed up cat food. He was really playfull, and was one of those cats that love attention, not the kind of kitten that is scared of people. I estimate he's about 2 months old, possibly 3 months old. This morning, my dog got the kitten in his mouth and started to shake and bite him! Now, the kitten seems to be doing better but i think his back legs are paralyzed. I lift them up gently and he doesnt even notice. I'm scared. My mom says she will call the vet but she is cheap an I doubt she will take him to get an appointment. PLEASE PLEASE PLEASE PLEASE HELP! I love my kitten and I want him to be happy!