Thank you for the prompt reply.
The fact that he is exhibiting this odd behavior does indicate that there is something wrong. The possible systems that could be involved include cardiac, neurologic, orthopedic and even musculoskelatal. The problems that is not always easy to rule in or rule out some of the possible disease conditions. Orthopedic conditions like arthritis is not usually intermittent in it's presentation, but it can be. Some dogs will manifest pain but dragging instead of using the affected leg. If there was neurologic damage due to the previous injury, one would not expect that to be intermittent either, but if there were for instance a vertebral disk that was intermittently bulging and putting pressure on the spinal cord, that could lead to loss of nerve function and pain in the hindlegs. There have been reported cases of focal seizures that occur with symptoms of episodic paralysis of the hindlegs.
Another possibility could be due to cardiac or circulatory issues. Intermittent claudication or lack of blood flow to the hindlegs has been reported in dogs. This can be exacerbated by exercise. It may be helpful to learn how to check his hindleg pulses so that on the next episode, you can make note of his circulation to those hindlegs. There is another disease, called myasthenia gravis, that should definitely be ruled out. In this disorder, there can be an intermittent paralysis due to the dog's own immune system producing antibodies that block the neurotransmitters that supply the skeletal muscles and the hindlimbs can be affected. Here is a link with more information about this disorder. LINK HERE
Even though his symptoms are intermittent, it still warrants a thorough physical exam and work up to see if a proper diagnosis can be determined.
I hope this is helpful. Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please reply so I can clarify for you.
My posted replies are for general education only and not meant as a diagnosis. Only after a thorough veterinary examination can a diagnosis for your pet be made and specific treatments be advised or medications be prescribed.