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Had your dog been scooting on his bottom at all?
Has he been biting at his back side?
Is he able to jump up?
Eating, drinking, and eliminating normally?
There are two things that come immediately to mind with the symptoms that he is showing. One being impacted anal glands causing discomfort so that he is holding his tail down. The other is a possible spinal or neck injury. I would check his anal glands to make sure that is not the issue. You can read about doing that below:
Click here: The Anal Sacs Page
If the anal sacks are ok, then I would consider an injury. You can try giving him a baby aspirin and keeping him calm to see if that helps at all. If it does help, you can do that every 12 hours until Monday when you can see your vet. If it doesn't help, then he needs to be examined so he can be properly diagnosed and treated. Let me know how you make out with him. :)