Thanks for the help guys- this is my first Sheltie and he's never had to have any kind of drastic haircuts (ie shaved for surgery) so I wasn't sure how long it would take to grow back, but your experiences are reassuring! Fletch was just at the groomer's two weeks ago so I can't take him back quite so soon due to money, plus it's super cold out and I don't want to cut the fur that remains even shorter. I guess we'll just have to wait it out.
As for gramps, I was really more upset about the fact that he went behind my back and waited until I wasn't around to do it (as if I wasn't going to notice when I came back???) than anything, but I'm not one to hold grudges so he has been forgiven ahah. And he's come to his senses about how ridiculous the cut looks, so I don't think he'll be picking up the scissors again any time soon.