Good morning...by Manx syndrome, I am assuming that you are referring to sacrocaudal dysgenesis where the ending vertebrae of the cat are fused and cause a shortening of the spinal column. This is an hereditary condition within the Manx breed and can cause gait abnormalities ("bunny hopping"), neurological deficits and incontinence/bladder infections.
DId your veterinarian take x-rays to confirm that this is the case? If so, sadly, there is no treatment other than supportive care. You can learn to manually express his bladder to help prevent urinary tract infections and incontinence. Similarly, providing him with stool softeners could help in case he would develop constipation.
Finally, please make sure that the breeder of this kitten is aware of this situation. Many Manx breeders are diligently working to eradicate this condition from their lines and the parents of this kitten should not be bred again.