Unwillingness to take a butt sniff is actually a sign that the dog needs more socialization. This is like a handshake for people, so try to encourage your dog to get used to it. You can feed your dog a treat while the other dogs gets a sniff.
OTOH, some dogs really stick their nose up there, which is rude. If your dog is getting a rectal exam, it's okay for him to object.
