Unfortunately ***** *****s as a breed are prone to a problem with their intervertebral discs, which are the spongy cushions between the individual vertebrae in their back and neck. These spongy discs can move or rupture and place pressure upon the spinal cord which can lead to pain, sometimes weakness, 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 severe weakness or paralysis then surgery by a board certified veterinary neurologist, as soon as possible, is indicated.
Ideally he would be examined by his veterinarian as soon as possible. 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. I understand that may be difficult given the weekend is coming.
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 the 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.
He might have discomfort bending over to eat and drink, so elevating his food and water may help too.
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 and weakness in dogs.
At home tonight you can also alternate between hot and cold packs for 10 minutes at a time. Warmth relieves muscle spasms and cold decreases inflammation.
Primary constipation is extremely rare in young, otherwise healthy dogs.
If you aren't sure that part of his discomfort isn't related to constipation (his stools seem especially dry or hard, and difficult to pass) you can add 1-2 tablespoons of canned pumpkin (not pie filling, just pumpkin) or Metamucil to his meals to increase fiber. Make sure to get him outdoors frequently to pass stools. Slow, short walks to keep the gut moving may help too.
If he is really having trouble 1/4 cup of milk can soften stools, as most dogs are lactose intolerant.
You can try placing pressure on his back muscles on either side of his spine with your flat finger tips, working from mid chest down to his tail base area. It can hard to get them to show pain because the pain may be deep and he may guard it by tightening his back muscles.
Most dogs will avoid stairs, or may position themselves oddly to do so with back pain.
Please let me know if you have any further questions.