Aww, what a sweetie Nate is
. Congratulations on his adoption, driver
I don't know if it's the same with cats as it is with de-worming dogs, but stools normally firm up within a week. I do know that more than one de-worming is necessary to kill all intestinal parasites (usually 2-3 weeks apart).
Chances are, if you switch him over to a good quality canned (check out our food forum here), his stools, and his health, will improve even more quickly.