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Hello. I'm sorry to hear about your dog.
It sounds like he is down in the rear limbs, but still has some strength in the front limbs. The most common cause of this in small breed dogs, is a disc herniation. A disc can herniate and press on the spinal cord. This leads to weakness in the rear legs in mild cases. We can see full paralysis in more severe cases.
A neurologic exam can be done to get a good idea of what's going on. To confirm a disc herniation and spinal cord compression, you would need an MRI or CT of his back. From there, surgery is done to decompress the spinal cord. Without surgery, the chance of regaining function is slim with a spinal cord compression and loss of movement/ feeling in the rear limbs.
I would get to a vet as soon as you can to get this diagnosed and then determine if an MRI or CT is needed.
Best of luck, let me know if you have any other questions.