My gold fish looks like his ribs are sticking out?

Is he ill? He's fed once a day n I've had him for 3 years almost n always fed him the same n he's always been fine. My child poured a whole tub of food in yesterday morn so I changed the water...it was really cloudy n smelt this morn so I've cleaned the small gravel and done a full water change. I know its nt good to do full water chances more thn once but my fish has been resilient to all torment n numerous water chances in the past. He's very loved!! Is he okay?