My dads dogs built up an intollerance to beef, which is very common. When he removed all the beef protein from their diet they stopped scratching. Each dog who has and allegy to protein is different so what is the main protein in what you are feeding now? If he likes that food, is there a different flavour? you could start there instead of changing his food totally, you may still have to and any brand who have the sensative skin formular is a good start.