Why is your elderly dog's butt smudged with poop?

My dog has the same problem; It could be a variety of things:

-he could have hip displasia and it might hurt to squat

-he could have a spinal disk problem, in which case he might not be able to feel his legs or tail (to move it to go to the bathroom)

-he might have Arthritis

My dog has all of these issues and I suggest making the dog soak in water, and then taking him to the vet. He may simply have problems with the muscles in his anus.

Hope it gets better!