My dog, a rescued grey...has been showing increased weakness in his back legs. He's lost 16lbs. I received a panic call from the kids the other day. I brought him in and he has 'canine degenerative myelopathy', or, sorta, a type of multiple sclerosis for dogs. He will have progressive loss of use of his rear legs, eventually requiring euthanasia. He is 12 1/2.
My faithful dog is also 12 1/2 and a large breed adoptamut. He gets an arthritis aspirin as needed for pain but I just started him on a product called Steadfast to help improve mobility. A little spendy for a large breed dog but it claims visible results in 7 - 10 days or your money back.
Take care.
Live your life while you are still living. Riding the trail less traveled.