This gait could be the result of hind limb ataxia or hind limb weakness. "Hind limb ataxia" (or uncoordination) happens when something interrupts the nerves responsible for proprioception (body awareness). The cat's brain doesn't receive this information and cannot coordinate its movements. This can be from a number of causes including spinal trauma (even if you can't feel anything down his back), infection, genetic abnormalities and many other disease processes. Equally, Hind limb weakness has many causes. Whatever the cause, this is quite serious and you really need to seek veterinary attention.