We took the turtle to a reptile vet, who said that despite his appearance, he seems to be in good health. He also said that the excess fat may be due to the fact that his prior environment might have been too cold, causing him to prep for hybernation. The turtle hasn't eaten or pooped since we've had him (for around 10 days), and we're still trying different foods, however, he's curious and active right now, hopefully we can release him into the yard soon.