Cats can develop neurological issues in the back after a tail amputation. And they can occur years later as it slowly gets worse. And like humans, cats can develop phantom pain in a missing limb or tail. Get him in to the vet and discuss it - drugs like Prozac can really help a lot. And if it is a real issue like not moving right due to the tail injury (even though it's many years later) the vet can prescribe drugs like gabapentin to control that.
Years ago I fostered a kitten who had gotten caught in the fan belt of a car. She had most of the skin stripped off both back legs, and her tail ended up being amputated after the joint next to her body. Amazingly she recovered, and had full bladder control (sometimes they can become incontinent when the injury is that 'high'). But years after, in her new home, she developed similar symptoms to your cat's. This was thankfully a very enlightened vet, who thought it was phantom pain. Prozac really helped her out! Give it some thought, and get him in to be seen by your vet.