Don't run him on pavement as that will be tough on the joints (as opposed to walking on soft ground like a dirt road). Keep jumping to a minimum. Swimming is also a great exercise for dogs with joint issues. Do not stop exercise altogether because having good muscle tone to support the weak joint is crucial.
Try this Glucosamine supplement, I gave it to my boy for a few months when he had an injury but it's good for joints in general so it could slow down the deterioration of the joint and reduce the pain.