Our ol' boy who had arthritis did well on buffered aspirin and Glucosamine/Chondrotin/MSM supplements.
I've since heard that Tramadol has fewer long-term side effects than aspirin. And, having a grandma who died when ulcers in her stomach bled out (due to long-term aspirin use and ill-advised blood thinners given in the hospital post-stroke), I might opt to keep aspirin use to a minimum.
I'd go with more home made bone broth + green-lipped mussel (GLM)
I have a GLM powder that both dog and cat adore - it has a very fresh ocean smell to it. It's also one of the few supplements the cat will eat and not puke back up.
I'll triple the vote for Turmeric, too. Very strong anti-inflammatory properties.
If he's not already on mega doses of fish pills or another source of omega-3's, you could up the dose and/or find a different source (for variety) - like sardines or mackerel (low-sodium tins or frozen).
I've heard wonders about massage therapy, too... so give that boy some love!