Hobbling and pooping is not near a good enough reason in my opinion to put your family pet down. Have you tried to give him medicine for the pain? Are they saying its just arthritis from old age or what? Almost everyone (especially anyone or anything older) lives with pain. It's just part of life and not a reason to put the dog down unless it's extream and there is no help for it. what you've explained so far doesn't sound extream. I know you want to help the dog but I don't think putting him down is the way to do it...at least not yet. Dogs can get around pretty well on three legs. The pooping may have to do with depression also if he just lost his life long friend and is now haveing physical troubles. You should try to comfort him and be there for him as much as possible and then reassess if things get worse.