If he still seems happy and enjoying life that is a good thing! Usually, we used to advise to put dogs down when the bad days outnumbered the good ones. Of course, this is a personal choice, so owners who know their dog best should decide based on their personal experience with their pet and perception of how much they are still enjoying activities such as eating, being pet and interacting with their owners. You can consult with your vet to see if there are any options for better pain management. There are also mobility harnesses to help old dogs get around.