First of all your labrador is extremely overweight. Ideal weight is around 30kgs and he is almost twice that. Here is a link for reference: https://www.dogspot.in/my-labr ador-is-overweight-what-should -i-do/
Because of obesity his legs and spine will become weaker till paralysis, especially at such a senior age. Decrease food intake. Increaase exercize and reduce about 20kjgs pls that would solve most issues.