If he's doing great otherwise, there's lots to try yet. If all else fails, having the vet simply change up his pain meds might make a difference to him. There's a lot of options that might get him more comfortable.
Have you tried Adequan therapy (injections)? It's not a pain med. It's a compound injected into the low back muscles that goes through the synovial fluid into the joints and lubricates them, sometimes even building new cartilage. In other words, it heals
the arthritis at the source. It had a near miraculous effect on my old dog, and I've seen it prolong the quality of life in several dogs in rescue too.
The only downside is convenience and cost. It requires 2 shots a week for 4 weeks, then you step down to maintain at every-other week, probably for life. We could tell when it was time for the next shot, as the stiffness came back. Some young dogs can maintain at monthly, but mine never could go past 2 weeks. For cost, you can get the cost way down by doing the injections at home, with your vet's permission (they typically want to do the first one or two at the clinic, to see if there's any reaction--then most vets I know are happy to have you continue it at home).
The cheapest place I know to order Adequan Canine is Valleyvet.com (requires your vet's RX). They also sell a similar generic called Chondroprotec, for horses. Some vets use it in dogs too--I have no personal experience with it, but it's about half the cost. If cost is an issue, that's a conversation to have with the vet. I think the most recent 65-pound dog we had in rescue on bi-weekly shots went through 2 vials of Adequan every 6 weeks, once she was on the maintenance dose (= about $120 per 6 weeks through Valleyvet, or less with a sale).
I would also add Natural Eggshell Membrane, Collagen II (or chicken bone broth, if you want to extract it yourself -- Gatorbytes and Carmspack have both posted how to make it), a big dose of fish oil (we worked up to about 4,000 mg/day and were headed higher with a big dog), Vit E (mixed tocopherols). You can get good prices on these things on Vitacost.com.
Here's my dog's journey: https://www.germanshepherds.com/forum...ur-senior.html
Once we got all the supplements and Adequan loaded, we were able to very slowly reduce and fade away his carprofen/rimadyl and he was more
mobile than he'd ben when on it. (We kept the gabapentin at night to prevent a pain "boomerang" effect that can undo all the progress.)
My biggest regret was not starting acupuncture sooner with him too. In once 20 min. session, the difference in his rear-end strength, balance, and stability was remarkable.