New puppy, bad diarrhea?

I think I already know the answer to this but need some reassurance. We got an american bulldog puppy a few days ago. The owner was feeding him cheap cheap off brand dog food and didn't give us any to use to switch him over to what we wanted to feed him (blue buffalo) After about a day and a half of him eating the blue buffalo he started having bad diarrhea and a little bit of vomiting. Now he won't eat food but he will drink water(for about 2 days now). Do you think this is just the quick switch over of dog food reaction?