There are a few things that I would consider in a dog this age.
1. He could have a patellar luxation.
This is a problem in some dogs in which the groove that the patella (kneecap) sits in isn't deep enough. With this problem, the patella can pop out of the groove and suddenly the dog cannot use the leg.
You can see this often if you watch Jack Russels running - you will see them hold up a hind leg for a few beats and then put it down again. This is a classic symptom of a patellar luxation, and then spontaneous return to the normal position.
BUT sometimes it doesn't pop back in.
In these cases, surgery is usually recommended to deepen the groove in which the patella sits. Has your boy ever had this "skipping" type behaviour on either hind leg in the past? If yes, then I would be very suspicious that this is the problem.
Here is more: http://www.petplace.com/dogs/medial-patella-luxation-in-dogs/page1.aspx
2. He may have stretched or ruptured his cranial cruciate ligament.
This is something humans get as well, but in humans it is called Anterior Cruciate Ligament.
THIS IS WHAT I FEEL IS MOST LIKELY FROM YOUR DESCRIPTION!
The cranial cruciate ligament is one of the structures that stabilizes the knee joint. It is prone to problems. If the dog is in motion, and the lower leg is held still (by going down a hole, getting caught on a string, etc) while the upper leg keeps moving forward, there will be damage to this cranial cruciate ligament.
The symptoms seen are usually a sudden onset of hind-end lameness, with toe touching seen at standing and walking, but the dog often carries the leg when running.
Here is more information about this problem:
And you can see more here in this video:
3. Your boy may have a fracture (broken bone) of one of the bones in the leg. Probably, with this problem your dog would yelp when you touch the leg.
I have seen this many times with a dog getting their toes jammed under the door of a crate, and then twisting somehow and causing a fracture, usually in the foot area. Symptoms are a sudden onset of non weight bearing lameness, just as you are describing. X-rays would help to diagnose this.
Here is more: http://www.petplace.com/dogs/fracture-of-the-metatarsus-and-metacarpus-in-dogs/page1.aspx
4. It is also possible that your dog has dislocated his hip.
Again, your dog would almost certainly yelp with you moving the leg around.
This is a common dislocation and again could happen if he trapped his toes under the door and then pulled. It is unlikely but certainly possible. With this, again you would have the symptoms of non-weight bearing lameness. Here is more:
So, in summary, there are a number of different orthopedic conditions that can all give these same symptoms you are describing in your dog.
In order to diagnose this, a veterinarian would have to perform a thorough orthopedic examination, and a neurological exam. The vet might well suggest an x-ray. For this part, it might be necessary to give your dog a pain killer to allow proper positioning for the x-ray without hurting him. I do think your dog needs to see his vet to have this looked at. Something is going on, and the sooner it is diagnosed and treated the better.
I definitely would recommend a visit to see your veterinarian. If your boy is eating and drinking normally, then you should be fine to wait until your family veterinarian opens. In the meantime, keep him as quiet as possible - just out to void and back in again.
Be particularly careful with him if he has to go UP stairs as this is when all his weight is on the back legs and he is more likely to do additional damage.
I very strongly suspect that your little guy has injured his cruciate ligament. Try to keep him as quiet as possible. If it is just a stretched ligament, then 2 weeks of complete rest (NO running, just out to void and back in, NO stairs) might allow it to heal. However, if he has completely ruptured it, then rest will have no effect and he may need surgery.
I hope that this helps you to help your dog!
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The above is given for information only. Although I am a licensed veterinarian, I cannot legally prescribe medicines or diagnose your pet's condition without performing a physical exam. If you have concerns about your pet I would strongly advise contacting your regular veterinarian.