yes, but has he had his immune system so low before – the skin itching (very common food reaction) will probably have been his immune system fighting something, then you gave steroids and antibiotics both knock immune system more until theres virutally nothing and then when you give him food (that he may be allergic to anyway) his immune system just says NO NO MORE !! puts its wee foot down so to speak ?
a dog with low immune is gonna have completely different body chemistry and abilities to process food for energy etc etc . . . to a well dog.