Sounds like hes against his new food, if its hard food you should try wetting it with water or gravy to see if that changes his mind, he might not like the feel of crunching it, My dog eats my other dogs poo Its truely disgusting but our vet said its safe and we just need to try and break the habit, Ive heard that feeding the dog pineapple or pineapple juice can detter them from the taste of poo but You need to check that out before you try as Im not 100% sure its safe or works, Some fruits can be dangerous to dogs in excess so I would check it just to be safe