Yep, a few days. I would try the live rat pup, at that point. If he's not interested in anything after a week, you may want to do the vet after all. A vet can tube feed him some Carnivore Care or something, and get some nutrition into him, and then after a few days you'll have a fecal sample to check him for parasites. As skinny as he is, he should be willing to eat as soon as he's adjusted to his new home, so if he doesn't, he may well be sick.