Just watch him for the day. Don't give him anything to eat right now, to allow his little tummy to calm down (water is alright though). Offer him either a small amount of his normal food, or some chicken and rice for dinner. How old is he? You just changed him from puppy to adult food? I have a friend who switched her dog from puppy to adult (same brand) and he also got sick. He is also a small dog and his digestive tract wasn't ready for the abrupt change. You may try a slow change, mixing 1/4 adult and 3/4 puppy, then slowly increasing the amount of adult food and decreasing the amount of puppy food.