Have you had his mouth checked at the vets, is he teething, some animals suffer more than others during this time, eating can be painful, then even when its not anymore they think it will. If you can rule all this out, have you ever thought he maybe playing you up, I know we are all concerned to ensure our animals eat well, but some of the brighter ones will work you, so you offer all sorts. When I was a teenager we had a cat that would only eat one brand of food, it was made by woolworths, when the one in our home town was out of stock, we were dispatched on the train to another branch to ensure Tessa would eat. My animals have it hard, they eat what they are given or go without. the trouble with fussy eaters is they don't get a balanced diet, the worst case I ever came across was a dog who would only eat Yorkshire puds and roats potatoes, with a bit of gravy of course.