Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that Smokie is now unable to use his rear legs properly.
Your fellow may have one of a few things occurring.
I am glad that he had a physical examination by a veterinarian and diagnostics such as radiographs of his spine. If they are concerned about a blood clot then an ultrasound of his heart, as well as cultures of his urine or blood and a complete blood count may help narrow things down.
Because he is outdoors unsupervised trauma to his spine is a possibility but it should leave some external signs, which you don't mention, and this makes this less likely.
Another possibility is diskospondylitis, a bacterial or fungal infection of the intervertebral disks and surrounding vertebrae. It is usually caused by an infection somewhere else that has been spread to the vertebrae by the blood supply. It is a painful condition. Treatment is usually antibiotics or antifungals for at least 8 to 12 weeks, sometimes as long as a year. Some of these pups require surgery to remove as much of the infection as possible and possibly stabilize the spine. Frequent re-evaluations by his veterinarian are necessary. Prognosis depends upon the amount of damage done and the organism causing the infection.
Intervertebral Disc Disease is the most likely cause of his symptoms however. Intervertebral disc disease is seen more in short legged, breeds with a long back. Weakness or paralysis happens when the spongy disc or disc material between the vertebrae slips up and compresses the spinal cord causing pain and weakness or paralysis. Radiographs can sometimes be diagnostic but often early on in the disease process, because the discs are soft tissue not bone, everything will look normal. An MRI is the best way of diagnosing disc disease. It can be treated with anti-inflammatories and rest or surgery depending upon how much function is lost and how much pain the patient is in. Prognosis is much better if the patient's sense of deep pain remains intact and if surgery is done quickly in the cases that require it. It does not usually cause a fever.
Fibrocartilaginous Emboli is a piece of a degenerated intervertebral disc that has broken off and lodged in a small artery or vein near the spinal cord blocking the blood supply to his spinal cord and degeneration of the cord. It is often one sided, only painful the first few minutes to hours, and doesn't cause a fever. Signs don't progress after 24 hours unless another emboli is thrown. Prognosis depends upon how much damage is done initially. Recovery is slow and gradual, the most improvement happens between day 21 and 42, and may not happen at all if his sense of pain perception is lost.
If the cause of his condition is an Aortic Thromboembolism (saddle thrombus) then this is a blood clot that formed in the heart, usually secondary to a primary heart disease called cardiomyopathy, and has lodged in his aorta usually cutting off the circulation to his rear legs. We see this much more commonly in cats than we do in dogs. This condition is very painful. Temperature of the affected limbs is sub-normal. His pads on her rear feet may feel cooler and look darker in color compared to his front foot pads due to poor circulation. Treatment is anti-clotting medication like heparin or low dose aspirin as well as pain medication such as tramadol and treatment of the primary heart disease that predisposed him to developing a clot. Prognosis can be very poor because of the damage done by the clot as well as the underlying heart disease. Recovery may take weeks.
At this point if your fellow has no evidence of heart disease and cold rear feet, trauma or a fever which would mean this is infection related then intervertebral disc disease is most likely. An MRI should be done if he remains unable to use his rear legs, and surgery may be needed, especially if he loses feeling in his rear feet.
Please let me know if you have any further questions.