Hi there,
I have had two with Manx Syndrome. I also have a tailless cat right now with so few vertebra that it amazes me she can function at all but she is "normal." She is past the four-month old mark so I am pretty confident that she will not develop symptoms. The four-month old mark is critical because Manx Syndrome can show up anytime UP TO four months. So when a stupid person who actually backyard breeds for taillness tried to sell a "healthy" kitten at 7 weeks, it does not mean the kitten is free from the Syndrome but is only asymptomatic at that time. Usually, though, once the symptoms present themselves fully, they do not worsen over time, but remain at a plateau.
Without giving false hope, I must note the opposite. I had two kittens come to me who were "born with" Manx Syndrome in that they exhibited all the symptoms as neonates. In both cases, by the time they were one, I no longer needed to diaper either of them. The Maine Coon mix, Harry, was able to use the box consistently. This is the kitten who went through ####: surgeries, hospital stays, enemas, stomach pumpings, etc. The other cat, Egg, never did have an anus that closed properly but with lactulose and then just diet, she did not leak feces at all.
In order to pass on advice, I will use what I did with Harry as an example. Harry was a Houdini (haha, I know). At one point, Harry the Maine Coon mix was actually so small (he died at about 20 lbs at age 4 from cardiomyopathy) that I used to buy the cotton mittens that protect babies from scratching themselves and snip two tiny holes for the legs. They had to be tiny because the cotton would stretch a bit and then the holes would be too big. I would line them with maxipads. I held them on to his body but vet wrapping the top, comfortably but firmly, around his waist. It worked well enough and I could wash the mittens as they were soiled. I did not like anything like that around his waist so I looked into another product. I highly recommend the following:
http://www.joybies.com/pagecat.html
These diapers have straps. I used to pin the back straps together in the middle of his back so that there was a "X". This took up the slack and made the whole outfit stay on better. Lastly and most importantly, I left Harry in a large cage overnight for his skin to air out. It is critical because any cat who is diapered 24/7 runs a very high risk of urine scald or fungal/yeast infections in damp skin areas. I think that now I would use Bag Balm (for livestock use but I put it on myself and my son all the time). I would make sure the skin is dry and then rub it around the anus and genitalia in order to provide a barrier from the urine that could accumulate in the diaper.
As for meds and trying to control the stool to some extent, lactulose or pumpkin will help to keep the stool firm. As well, a natural way to get firmer stool, I have found, is to feed raw.
Hope all this helps. I love tailless cats.
Nothing but the following product could keep diapers on him.