Anything you can do for dog who is otherwise healthy.. but has trouble walking and using back legs?

He is almost 13 years old.. back legs started to shake about 4 months ago.. I took him to vet.. he said it was probably arthritis and said if there was pain to give him an aspirin. There is rarely pain but just recently he started having accidents (just poop) and just yesterday he has started having problems standing/walking. he kinda hobbles.. or just drags his back side. Is there any cure for this? Im afraid of what the vet will tell me and he is not in any pain.. still very happy dog.