Hello, and thanks for writing in.
I'm sorry to hear about your dog's problems. This is not uncommon in this age of a dog. I commend you on wanting to keep him as comfortable as possible. There are many ways to treat pain and arthritis (or suspected arthritis) in older dogs. By far, the safest and most effective is to use Non-Steroidal Anti-Inflammatory Drugs (NSAID's) prescribed by your veterinarian. They work well for pain and inflammation. Examples include Rimayl, Previcox, and Metacam. There are other pain medications that can be prescribed, and they include Tramadol, Gabapentin, and Amantadine. They can be given at the same time as an NSAID. It's also a good idea to have him on a dog glucosamine product, such as Synovialflex chews, or Cosequin DS. (which it sounds like you are doing). There is also an injectable medication called Adequan that is very effective, even in dogs that don't seem to respond well to the other mentioned medications. Overall, these products are quite safe. The NSAID's have the potential to cause stomach upset, vomiting, diarrhea, or stomach ulcers, but the incidence is low.I would not recommend giving any over the counter human pain medications, especially Tylenol, Aleve, or ibuprofen, as these can all be toxic to dogs and can cause significant problems. Buffered aspirin is the only human pain medication that is even considered somewhat safe in dogs at 5mg per pound of body weight, given every 12 hours. But even aspirin has the potential to cause vomiting, diarrhea, and stomach ulcers (more so than the dog NSAID's mentioned previously). So, if you do give aspirin, give it with caution, and stop immediately if he develops any abnormal symptoms.I encourage you to read a good article on treating arthritis in dog HERE.
I hope this helps.