There's this dog by my house who can't walk with his 2 back legs so he drags himself everywhere it seems to me like maybe a car hit him he looks so skinny and helpless I don't know what to do...

Yes, he may have. Can you put some food out, hamburger or scraps or dog food, when you see him coming?
Look in the phone book for the ASPCA, a shelter, or the dog warden in the blue pages under your city name.