Apparently he has cancer of the kidney. According to the vet it is fairly common in Budgies. And the cancer is pressing against his spine causing the paralysis. He is still able to move around the bottom of the cage, eat, drink and fly but probably live more than a few months. The vet also said that is is unlikely that he is in any pain but he will be towards the end and I should bring him back in then. Not happy news but the budgie is adapting rather quickly to his new lifestyle. Something more akin to that of a quail.