When should not eating become a concern ?

  I would worry if he goes more than a day without eating. I know you've recently started him on pred; that usually increases the appetite.

Fox needs to EAT while taking the pred because it can be hard on the stomach.  Try something simple -- like ground beef and plain white rice (cook the rice way beyond normal with lots of water and add the meat last.  Not expensive ... but it may help Fox eat and it's simpler to digest. 

But yes, you don't want Fox to stop eating  ... the pred will make him drink, but depending on how he feels, some dogs will not eat if they feel awful.  But you have to prompt him to eat -- no big bowls of food, but a small amount in a saucer.  You can even put a tiny bit of garlic **powder** (not garlic salt, but either plain mashed fresh garlic - just a tiny bit, or a little bit of plain garlic powder) -- might entice him to eat.