Watched the vet do the pm - really interesting!
She took out the spinal cord, but couldn't see any abcesses - mind you, with the blood etc, it is quite difficult! She also looked at the brain. She still thinks some sort of micro abcess on the spine as he lost use of both front and back legs.
It is caused by bacteria getting in - either from the navel at birth, a tick bite or even a scratch in the wrong place - so just unlucky I guess.