Welcome to Just Answer. I'm very sorry to hear about this guy's paralysis of his rear legs. In all honesty, the most common cause of rear leg paralysis is a spinal cord compression. This occurs due to a disc that is compressing the spinal cord. At this time, this is very serious and needs to be evaluated by a veterinarian. I would get him into an ER clinic now to have him diagnosed and treatment options given. Here is a good link on this type of injury.