Thanks very much for the response.
I am sorry to hear that you came home to find him this way, not a great way to start the weekend.
Because his appetite has been normal and he isn't scooting but is hesistant to jump I highly suspect that we are dealing with back pain.
Unfortunately Shih tzus as a breed are prone to a problem with their intervertebral discs which are the spongy cushions between the bones of the back. These spongy discs can move or rupture and place pressure upon the spinal cord which can lead to pain, and in severe cases 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.
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 would see a veterinarian for an examination and some prescription medications, usually an anti-inflammatory, like Metacam, Rimadyl or Deramaxx but I highly recommend something for pain too, such as Tramadol. And if he is having painful muscle spasms then a muscle relaxant such as methocarbamol.
I understand that may not be possible tonight.
Starting now he should be closely confined. 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. You will need to confine him for several weeks, even as he starts to feel better or he may reinjure himself. When you take him out to eliminate make sure he is able to pass urine and stool without any problems.
You can also use a heating pad set on low to medium on his back for 5 to 10 minutes a few times tonight. This will help decrease the painful muscle spasms that go along with back pain.
Then I highly recommend that you have him examined tomorrow.
I also recommend using a harness rather than a collar in dogs with a history of back pain as they distribute forces more evenly and are less likely to lead to neck pain as well.
Keeping him on the thin side is recommended to decrease stress on his back.
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 if the most common cause of back pain in dogs.
Please let me know if you have any further questions.