Still catching up on this case.
I was thinking the same thing, wouldn't dog saliva (and maybe dog blood) have coated his arm with ample DNA to test?
I reviewed the autopsy photos and compared them to forensic dog bite images. I have to say, they look very similar to me. I guess the canines are usually the most prominent injuries because an arm is a cylindrical surface. They look like pairs of puncture wounds with trail marks, all in the same direction. They could measure the distance between the canine marks left on the victim and compare it to the dogs mouth measurements.
If the dog is not around anymore, they can't cast the dogs teeth and match it to the injuries.