I knew of a manx kitty who had defecation and walking problems with his hind legs, but he was an outdoor barn kitty owned by a vet-tech and she was able to monitor him. I didn't know him very well...
My own kitty is having difficulty with his back end, both elimination and back leg function, though his tail seems fine. Reilly's problem is degenerative and progressive. We will eventually lose him...but for now, he is happy with himself.
He has difficulty urinating and defecating.
To defecate, he lays on his side and pulls his back legs up to his belly and 'paddles' them until he is finished. Sometimes he vomits from the effort of elimination.
To urinate, he also lays on his side and it seems he has to strain very hard to get the urine past his sphincter muscles. His tail shakes with the effort.
Your kitty sounds like he has no control or feeling of when it is time for him to go. Your vet should have shown you how to express his bladder when he was in last time. I'm sorry, I don't know what to advise you...
Best of luck to you and your kitty,