I'm no vet, nor am I very experienced with dogs (the canine kind), but I just have a gut feeling that it is arthritis or a disk problem, or both. If it is, then cortisone epidurals, followed by NSAIDs and rest should help, but watch out for gastric irritation. The doggie wheelchair may not be a bad idea, if it takes the stress off of the affected area and allows it to heal. He may not need it permanently. Consult with your vet.
We humans have a propensity for developing lower back injuries, but we have some advantages over quadripeds. As part of our evolution of upright posture and bipedal locomotion, we developed very strong trunk and abdominal muscles; these are all but absent in animals that walk on four legs. Our body parts are all arranged in vertical fashion to be centered and balanced on two relatively long and strong legs. The human spine functions as a vertical column, and it is very resistant to the pull of gravity that would cause it to collapse upon itself. The body layout of a quadriped, such as a canine, is all horizontal, with the body parts all hanging in a sack underneath the spine. The spine in four-legged creatures acts sort of like a suspension bridge, spanning the horizontal distance between the shoulders and the hips, with extensions to support the head and to supply a tail.
Muscles in the animal's back act as the suspension cables for this bridge, keeping it straight and in line. Canines, like people, lose strength and muscle mass as they age. So as dogs get older, the muscles that suspend this spinal bridge get weaker, causing the bridge to sag. Because the backbone in all vertebrates also contains the spinal cord, this sagging can cause some pressure or pinching of the spinal nerves that exit the backbone between each vertebral member. This is a common cause of spinal arthritis, and all quadripeds, including dogs, will develop arthritic backs eventually. In dog breeds that have long spines, the strength needed to keep the spine straight and level is greater, and so the inevitable weakening of the stabilizing muscles has a greater or at least more rapidly developing impact on the health of the spinal nerves. Dogs with relatively short legs have less bone mass and muscle and connective tissue mass in their legs to absorb the shocks and stresses of normal activities like walking and running. So the combination of short legs and a long spine is a sure recipe for trouble for a quadriped. Guess which dog breed is known for having a long spine and short legs?