I'm sorry to hear about Mosby. He sounds very uncomfortable.
Dogs that have back pain/inflammation will be painful with pressure and the nerve roots from that inflammation can be affected such that we may see a limp. Dogs with back pain display the sort of behavior he is, either because it hurts to use that leg due to nerve root pain, there are muscle spasms, or there is weakness.
This sounds like a problem with his intervertebral discs, which are the spongy cushions between the individual vertebrae in their back and neck. These spongy discs can move or rupture with just the wrong movement, or repeated movements, and place pressure upon the spinal cord which can lead to pain, and in severe cases paralysis. Perhaps he did a lot of jumping in the kennel, or laid on a hard surface or slipped on a smooth surface and hurt himself that way.
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.
If the dog is painful but has no evidence of paralysis we can try strict rest, anti-inflammatories and pain medications for several weeks to allow healing.
If there is evidence or weakness or paralysis then surgery by a board certified veterinary neurologist, as soon as possible, is indicated.
Ideally he needs an appointment with his veterinarian. If this is indeed a disc problem your veterinarian can prescribe a steroid or nonsteroidal anti-inflammatory to relieve pressure on his spinal cord and nerve roots, as well as something for pain too, such as Tramadol. And if he is having painful muscle spasms then a muscle relaxant such as methocarbamol as well.
He should be closely confined starting now. No stairs, running or jumping. If you have a crate for him I highly recommend using it. The less he moves around the more comfortable he will be and the faster he will heal.
He should go out on a leash to relieve himself. Do not use a collar for him, a harness which more evenly distributes forces if he pulls on his leash is better. You will need to confine him for several weeks, even as he starts to feel better or he may reinjure himself. Keeping him on the thin side is recommended to decrease stress on his back, but is no guarantee that he won't have another episode. Once a dog has one bad disc the likelihood of another is very high.
If you are interested in reading more here is a link to an excellent article about intervertebral disc disease, its causes and therapy: http://www.petwave.com/Dogs/Dog-Health-Center/Bone-Joint-Muscle-Disorders/Intervertebral-Disk-Disease/Symptoms.aspx
There are other less common causes of back pain such as infections, tumors of the vertebrae or the spinal cord itself or fibrocartilagenous emboli but far and away disc disease is the most common cause of back pain in dogs.
Please let me know if you have any further questions.