sorry I really don't know verymuch abt manx syndrome. I would think though that this isn't a progressive condition, it usually shows up as chronic constipation.
I found only one small mention of it on a site I frequent. sorry I can't be of more help...
http://www.petplace.com/cats/structure-and-function-of-the-brain-and-spinal-cord-in-cats/page2.aspx
I did find this, though:
http://www.vetlocator.com/askapetpro/special-needs-kitten-manx-syndrome/
http://www.vetlocator.com/askapetpro/special-needs-kitten-manx-syndrome/
http://www.catforum.com/forum/36-cat-chat/131376-manx-syndrome.html
http://www.youtube.com/watch?v=xMp4dCjzI08
I did a Google search and thought these things offered the best information. Sadly, I don't think there is any way to help his legs. That last one is on YouTube from someone who went through what you're going through now. It sounds like it would be wiser to get the kittens when they're much older to HOPEFULLY ensure that your heart doesn't get broken like it is now by finding out after you've fallen in love with your beloved companion cat.