There are a few causes of sudden onset of difficulty with the hind legs.
1. Intervertebral disk disease- These are the "shock absorbers" between the bones
of the spine.
Sometimes they will slip or explosively move upwards impacting the spinal cord and causing inflammation. The inflammation interferes with nerve signals to the legs. If only his hindlegs are affected, then usually the problem lies between the 3rd thoracic (beginning of rib cage) and 3rd lumbar (just beyond rib cage) vertebra.
Paralysis of the hindlegs indicates that the inflammation is severe and the longer it is left unaddressed, the more damage will be done to the spinal cord.
2. Diskospondylitis- this is the result of an infection (bacteria or fungus) within the vertebra and/or intervertebral disk. These dogs are very painful, and usually have a fever. Large breed dogs are most often affected. Antibiotics, pain medication, and intensive supportive care are required.
3. Fibrocartilaginous emboli- a piece of material (usually disk) breaks off and causes injury to the spinal cord. These dogs often will improve on their own with appropriate supportive care.
Some dogs are also suspectible to a condition called lumbosacral stenosis, which results in a narrowing of the spinal column and compression of the nerves of the lower back.
A tumor pressing against the spinal column would cause a similar issue.
Ideally, an emergency vet visit is recommended in order to figure out what is wrong and trying to get him up again.
The vet will likely take x-rays, and then discuss treatment options based on the diagnosis. The sooner you have him seen, the better chance he will have.
If there is absolutely no way that you can take him to a vet, then providing supportive care and waiting is really all that can be done. He needs a large amount of padding to help prevent rub sores on pressure points like the hips and knees.
His hind end should be rotated every couple of hours so as to avoid too much pressure on one side of his body for an extended period of time.
He will need to be kept clean with pee pads to collect urine and stool, and spot washing as need to prevent urine scalding or fecal material build-up.
His food and water will need to be brought to him.
If he is not too painful, you can do some physical therapy to prevent muscle
and joint contracture (inability to bend). Passive range of motion exercises to gently flex and extend each joint to promote joint fluid production and keep the joints limber.
Lifting him up with a sling is ill-advised at the moment, since we don't know the cause of the problem. If it is a disk, this could result in worsening of the condition.
Any insult to the spinal cord is best addressed immediately to enhance the chances of a favorable prognosis. If possible, Battles should be seen on an emergency basis.