I would probably get the vet to check Dio out if he doesnt perk up soon. Is there any chance he could have eaten something nasty? I am not convinced that it is the new food that is the problem, normally they get an upset tummy within 24 hrs if that is causing it - in my experience. He could have something like gastroenteritis (sp?) which will give his a very upset stomach. If you put your ear to his belly you can hear the stomach making some very strange noises. Is the any mucous or blood in his poo? If so, i would be straight to the vet. Good luck, i hope he has perked up. Jeep us posted on his progress.