I agree with KMG- looks like feces to me also. Regurgitated rodents smell so bad it makes us want to hurl too- so if this didn't do that, it's stool.

Snakes often conserve water by not going after every meal- they save it up until they really need to take a dump.

Sometimes snakes let loose inside their old skin, while they're removing the shed (talk about "neatness", lol); this is because the old skin apparently feels a little tight & the motion needed to shed their skin seems to "inspire" the release of stool. Your guy couldn't quite wait that long to go. Maybe next time?