my new dog is underweight

Question
I got a new dog yesterday and he is so skinny that even with his long fur you see and feel ALL his ribs and bones it's so gross. he is almost 2 yrs old and he is a husky/border collie mix. his previous owners left food out for him to eat as much as he wanted but they feed him the cheap food and have never brought him to the vet. i've got a vet appointment for him and it's not for a few days so until then i need to know what to expect may be wrong with him and what to do with him until then.

Answer
It's hard to say what could be wrong with him - it could be anything from malnutrition to cancer.  If you can, I would switch him from what he has been eating to whatever food you want to feed him over the next couple of days.  Even if it is a poorer quality food, a couple more days shouldn't hurt him, and it's better than him getting diarrhea from an upset stomach.  Definitely make sure to keep him away from any other pets you may have until your vet can rule out anything that is contagious.  If he has vomiting or diarrhea, is very lethargic then I would take him to the vet as soon as possible.  Check the color of his gums - they should be moist and pink.  If they are dry and tacky and very pale or white in color, he needs medical attention very soon.  I hope he gets better.

Amanda