In my opinion, your money would be better spent going to the vet for an evaluation and treatment options for pain management and inflammation such as prescription meds.
Has he had a senior workup? If not, that would be a good place to start.
Aqua therapy which is offered at some veterinary clinics as rehab, swimming in a pool or walking in water on a treadmill a few times a week can help a lot. https://www.vetinfo.com/aquatic-therapy-for-dogs.html
Supplements are not only most likely ineffective but they often have side effects.
Perhaps you will find some helpful information here http://skeptvet.com/Blog/?s=mercola
and http://skeptvet.com/Blog/?s=glucosamine
This is not veterinary advice; consult your veterinarian.
