How long has he been dragging his feet?
Not want to bring up a terrible subject but the course of DM (in both of our pups that had it) is usually only 6-9 mo till it has affected chest and etc. muscles.
Also DM is absolutely painless for the dog - they lose control of muscles as the disease progesses up the spinal cord from tail toward head.
Best of luck to and your boy! That is a terrible disease for dog and owner to have to go through. Your boy has a tremendously loving owner, it is clear!