Hi Emma, a change of food sometimes does it for dog and gives them a wee upset, you've now got his normal food back so I'd advise starving him for 24 hours then starting back with something light like scrambled egg for breakfast the morning after the 24 hours, some nice rice a chicken for his evening meal then gradually over the course of two or three days add his normal food into some rice all the time watching his toliet, he should be back to normal after 5 days and on his normal portion of dog food...good luck, let us know how he gets on!