It sounds as if he may have herniated a disk in his back, which is a very serious condition. The increased defection and loss of control can be due to nerve impingement causing decreased function and severe pain. The second possibility is that he may be having seizures. Either way he should be taken to a vet.
If it is has a herniated disk he may need emergency treatment to keep him ambulatory. Serious untreated disk disease can result in paralysis because the spinal cord can be completely impinged, and is often irreversible.
Take him to an emergency vet now!!