Deep breath. In humans, many people with this very condition live active lives that are relatively pain free through diet, thoughtfully designed exercise programs (core training, water exercise), massage, acupuncture and other modalities. I'll bet dogs can do even better. Given that you spent the money on the MRI, which is already more than a lot of owners would do, I'm confident you'll be able to figure out how to give him a full, fun life with minimal or no pain.
Maybe your vet would let you put the dog in a pool to drain off some energy? Perhaps under supervision of a doggie PT specialist? Not cheap, but the dog would likely love it, if the vet would okay it and do a referral.
ETA: my senior is on gabapentin. It can make him sleepy, so we were instructed to give it at night. It's good stuff, and less risky over the long-haul than NSAIDs.