Welcome to the site!
Deramaxx no longer available - alternatives that work for your senior?
Has some good information and links about adequan, etc.
It could be something with his back.
It could be something like a tick disease - if it were me, I would probably do a senior super chem/full blood panel (if it hasn't been done in the past few months) a heartworm/tick test (and maybe a send out tick test depending on where you live - or just do a run of Doxy? I don't know), a fecal and if you want to do a urine test too - sometimes UTIs can cause issues in weird ways.
There are also veterinary physical therapy places, acupuncture, laser, all sorts of things and many are listed in the links in that link. But it will depend on if it's something causing and what that is.
You can also ask your vet if something like this harness would be okay to use:
Canine Orthopedic Equipment Designed for Increased Mobility and Extra Support - Whole Dog Journal Article
the Ruffwear webmaster. I have used it on 2 seniors, much like a suitcase! It helped them to get in and out of the car easily, up and down stairs, and up from a down position easily.
Hopefully others will chime in.
Good luck and let us know what happens!