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: http://www.dogspot.in/my-labra dor-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.