This happend to us too. we switched our puppy from a really crappy food the breeder was using to Natures Variety and he had a bad reaction. We came home on the 3rd day and there was throw up ALL over the house and he has liquidy diah
. He was covered in throw up and was all sticky. We felt so bad for him. I called the vet and they had us give him a 48 hour "bland diet" with boiled chicken with rice or boiled ground beef with rice. NO reg food only that mixture. It helped instantly. After the 48 hours I started adding both his new and old food to the rice and meat mixture. It was so awful so see our little guy like that, especially when it was OUR doing.
I was more concerned about him not eating that bad food that I weened him too fast.
I hope your little one feels better very soon. You definetly do not want him to get dehydrated, that is very scary