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Hello. I'm sorry to hear about your dog.
It sounds like your dog is paralyzed in the rear legs. This is usually due to a disc herniation in the back. Given that he's a cow dog, a vertebral fracture or luxation secondary to trauma is also possible. X-rays are needed to rule out fractures or luxations. If it's a disc herniation, an MRI is needed to confirm, as x-rays don't show the discs or spinal cord.
No matter what, this is likely going to require surgery to get function back. Surgery is done to stabilize the spine or remove any compressive disc from around the spinal cord. As long as the cord itself is intact, surgery has a good prognosis for full return to function. Without surgery and with absent movement/ motor in the limbs, the prognosis is poor for return of function.
Here is a nice write up on disc herniations and the associated surgery:
I hope this helps, let me know if you have any other questions.