Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am very sorry to hear about Tyson's discomfort.
There are a few things that may be causing what you are seeing.
The first would be spinal arthritis which can cause inflammation and pain of the spinal nerves. These dogs have obvious spinal arthritis on radiographs and are painful upon examination. They may respond to prescription anti-inflammatories (steroids or nonsteroidals like Deramaxx, Metacam, or Rimadyl), pain medications like Tramadol as well as omega 3's and glucosamine/chondroitin supplements like Dasuquin or Cosequin DS.
It is very possible that he is dysplastic. Sometimes it is simply too painful to get up and move around. Symptoms can happen suddenly if a piece of the arthritic changes in his hip breaks off and is free in the joint.
Another condition that could cause rear leg weakness would be a degenerative myelopathy. This is a degenerative condition affecting the nerves of the spinal cord, it starts in the back legs and ascends eventually affecting nerves to the bladder and rectum which leads to loss of continence. These dogs aren't painful. They are uncoordinated in the rear because they have lost the ability to feel where their rear legs are and place them properly. They eventually become incontinent because they can not feel the stool building up and because their anal sphincter becomes loose. Unfortunately we do not have any good therapies for this condition. Radiographs look normal and diagnosis is by a blood test to look for genetic markers of the disease.
Another disease process would be lumbosacral stenosis and instability. This is an instability between the sacrum (pelvis) and the lumbar spine. It leads to inflammation and scar tissue and places pressure on the spinal cord leading to loss of function in his rear legs and stool and urine incontinence. These dogs are painful and should respond at least somewhat to steroids or anti-inflammatories. Diagnosis is by an MRI of the area. Treatment is surgery but once fecal and urine incontinence occur we cannot always reverse the damage done to nerves. Definitely worth trying though.
Unusual causes can also be a spinal cord tumor or a mass of the vertebrae or supporting tissues.
If he is an intact male, was ever used for breeding and is now running a fever Brucellosis is a possible cause as well.
A bacterial infection of the bones of the spine that spreads from a urinary tract infection or a puncture wound is another possibility, but those dogs are running a fever and are extremely painful and feel horrible.
Tick borne diseases like Lyme disease are another possible cause of pain and stiffness.
In order to help Tyson we really need to know why he is the way he is. An examination and radiographs of his spine are a great place to start, and blood testing for tick borne diseases should also be done if you live in an area with lots of ticks.
You can use prescription drugs from his veterinarian along with the omega 3's and glucosamine/chondroitin if arthritis or hip dysplasia is diagnosed. These nutraceuticals help improve cartilage and joint fluid health as well as reduce inflammation.
I recommend using a combination of a glucosamine/chondroitin product (examples are Dasuquin or Cosequin) and an omega 3 fatty acid (like 3V Caps or Derm Caps). I recommend an omega 3 fatty acid dose based upon the EPA portion (eicosapentanoic acid) of the supplement as if we do that the rest of the supplement will be properly balanced. Give him 20mg of EPA per pound of body weight per day. For example an 80 pound dog could take 1600mg of EPA per day. Omega 3's and glucosamine/chondroitins work synergistically and improve cartilage health and joint fluid quality and quantity as well as reducing inflammation. They can take several weeks to see full improvement but some dogs do very well with them alone. They are available over the counter.
Another option is a product called Duralactin. This is an anti-inflammatory product derived from milk proteins and it also has omega 3 fatty acids incorporated into it which can be very helpful. See this link for further information: http://www.duralactin.com/products_canine.html
In the meantime try and keep your pup quiet. If he has any spinal instability the more he does, especially jarring activities like running and stairs, the faster the condition can progress.
Please let me know if you have any further questions.